Fix bug in the descriptor parsing fuzzing harness (descriptor_parse) by making sure secp256k1_context_verify is properly initialized (via ECCVerifyHandle).
Background:
When fuzzing Parse(…) with libFuzzer I eventually reached the test case combo(020000000000000000000000000000000000000000000000000000000000000000). That input triggers a call to CPubKey::IsFullyValid() which in turns requires an initialized secp256k1_context_verify.
The fuzzing harness did not fulfil that pre-condition prior to this commit (sorry, my fault!) :)
Before:
$ mkdir descriptors/
$ echo -n 'combo(020000000000000000000000000000000000000000000000000000000000000000)' > descriptors/input
$ UBSAN_OPTIONS="print_stacktrace=1:halt_on_error=1" src/test/fuzz/descriptor_parse -runs=1 descriptors/
…
pubkey.cpp:210:38: runtime error: null pointer passed as argument 1, which is declared to never be null
secp256k1/include/secp256k1.h:305:3: note: nonnull attribute specified here
[#0](/bitcoin-bitcoin/0/) 0x561c032ccf25 in CPubKey::IsFullyValid() const src/pubkey.cpp:210:12
[#1](/bitcoin-bitcoin/1/) 0x561c022139c3 in (anonymous namespace)::ParsePubkeyInner(Span<char const> const&, bool, FlatSigningProvider&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&) src/script/descriptor.cpp:674:24
[#2](/bitcoin-bitcoin/2/) 0x561c02207680 in (anonymous namespace)::ParsePubkey(Span<char const> const&, bool, FlatSigningProvider&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&) src/script/descriptor.cpp:730:42
[#3](/bitcoin-bitcoin/3/) 0x561c0220080e in (anonymous namespace)::ParseScript(Span<char const>&, (anonymous namespace)::ParseScriptContext, FlatSigningProvider&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&) src/script/descriptor.cpp:774:23
[#4](/bitcoin-bitcoin/4/) 0x561c021ffb07 in Parse(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, FlatSigningProvider&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, bool) src/script/descriptor.cpp:994:16
[#5](/bitcoin-bitcoin/5/) 0x561c0218d5d4 in test_one_input(std::vector<unsigned char, std::allocator<unsigned char> > const&) src/test/fuzz/descriptor_parse.cpp:20:9
…
$
After:
$ mkdir descriptors/
$ echo -n 'combo(020000000000000000000000000000000000000000000000000000000000000000)' > descriptors/input
$ UBSAN_OPTIONS="print_stacktrace=1:halt_on_error=1" src/test/fuzz/descriptor_parse -runs=1 descriptors/
…
Done 2 runs in 0 second(s)
$
