Fix uninitialized variables in ecmult_multi test #759

Fixes #756

ACK 2e7fc5b5372067ecd33b2304e8c88ed6de98ff13 I inspected the diff. I did not test it and I did not check whether if makes the warning go away

tACK 2e7fc5b5372067ecd33b2304e8c88ed6de98ff13

Before:

$ gcc --version
gcc (GCC) 10.1.0
$ CFLAGS=-O3 ./configure
...
Build Options:                                                                                                         
  with endomorphism       = no                   
  with ecmult precomp     = yes                 
  with external callbacks = no                                                                                         
  with benchmarks         = yes                  
  with coverage           = no                          
  module ecdh             = no
  module recovery         = no           
                                                           
  asm                     = x86_64     
  bignum                  = gmp                                                                                        
  field                   = 64bit
  scalar                  = 64bit          
  ecmult window size      = 15                        
  ecmult gen prec. bits   = 4                         
                                                           
  valgrind                = yes                       
  CC                      = gcc                                                                                        
  CFLAGS                  = -O2 -fvisibility=hidden -std=c89 -pedantic -Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes -Wno-unused-function -Wno-long-long -Wno-overlength-strings -W -O3
  CPPFLAGS                =                                                                                            
  LDFLAGS                 =                           

$ make
  CC       src/bench_verify-bench_verify.o                                                                             
gcc -I. -I./src -Wall -Wextra -Wno-unused-function -g -O2 -c src/gen_context.c -o gen_context.o
gcc -Wall -Wextra -Wno-unused-function -g -O2  gen_context.o -o gen_context
./gen_context                                      
  CC       src/libsecp256k1_la-secp256k1.lo
  CCLD     libsecp256k1.la
  CCLD     bench_verify
  CC       src/bench_sign.o
  CCLD     bench_sign
  CC       src/bench_internal-bench_internal.o
  CCLD     bench_internal
  CC       src/bench_ecmult-bench_ecmult.o
  CCLD     bench_ecmult
  CC       src/tests-tests.o
src/tests.c: In function ‘test_ecmult_multi_batch_single.constprop’:
src/tests.c:2716:9: warning: ‘sc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 2716 |     *sc = data->sc[idx];
      |     ~~~~^~~~~~~~~~~~~~~
src/tests.c:2717:9: warning: ‘pt’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 2717 |     *pt = data->pt[idx];
      |     ~~~~^~~~~~~~~~~~~~~
src/tests.c:2717:9: warning: ‘*((void *)&pt+16)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
src/tests.c:2717:9: warning: ‘pt.x.n[4]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
src/tests.c:2717:9: warning: ‘*((void *)&pt+40)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
src/tests.c:2717:9: warning: ‘*((void *)&pt+48)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
src/tests.c:2717:9: warning: ‘*((void *)&pt+64)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
src/tests.c:2717:9: warning: ‘pt.y.n[4]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
src/tests.c:2717:9: warning: ‘*((void *)&pt+88)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
src/tests.c:2717:9: warning: ‘pt.infinity’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  CCLD     tests
  CC       src/valgrind_ctime_test.o
  CCLD     valgrind_ctime_test
  CC       src/exhaustive_tests-tests_exhaustive.o
  CCLD     exhaustive_tests

After:

$ gcc --version
gcc (GCC) 10.1.0
$ CFLAGS=-O3 ./configure
...
Build Options:                                                                                                         
  with endomorphism       = no                   
  with ecmult precomp     = yes                 
  with external callbacks = no                                                                                         
  with benchmarks         = yes                  
  with coverage           = no                          
  module ecdh             = no
  module recovery         = no           
                                                           
  asm                     = x86_64     
  bignum                  = gmp                                                                                        
  field                   = 64bit
  scalar                  = 64bit          
  ecmult window size      = 15                        
  ecmult gen prec. bits   = 4                         
                                                           
  valgrind                = yes                       
  CC                      = gcc                                                                                        
  CFLAGS                  = -O2 -fvisibility=hidden -std=c89 -pedantic -Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes -Wno-unused-function -Wno-long-long -Wno-overlength-strings -W -O3
  CPPFLAGS                =                                                                                            
  LDFLAGS                 =                           

$ make  
  CC       src/bench_verify-bench_verify.o
gcc -I. -I./src -Wall -Wextra -Wno-unused-function -g -O2 -c src/gen_context.c -o gen_context.o
gcc -Wall -Wextra -Wno-unused-function -g -O2  gen_context.o -o gen_context
./gen_context
  CC       src/libsecp256k1_la-secp256k1.lo
  CCLD     libsecp256k1.la
  CCLD     bench_verify
  CC       src/bench_sign.o
  CCLD     bench_sign
  CC       src/bench_internal-bench_internal.o
  CCLD     bench_internal
  CC       src/bench_ecmult-bench_ecmult.o
  CCLD     bench_ecmult
  CC       src/tests-tests.o
  CCLD     tests
  CC       src/valgrind_ctime_test.o
  CCLD     valgrind_ctime_test
  CC       src/exhaustive_tests-tests_exhaustive.o
  CCLD     exhaustive_tests