refactor: Use && in boolean context to avoid compiler warning #1006

This fixes a compiler warning:

./src/ecdsa_impl.h:312:12: warning: use of bitwise '&' with boolean operands [-Wbitwise-instead-of-logical]
    return !secp256k1_scalar_is_zero(sigr) & !secp256k1_scalar_is_zero(sigs);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                           &&

I didn't try to reproduce this, but I randomly saw it in logs on a 32-bit compilation with clang-14 (trunk) and some odd compiler flags like -O0.

Ah, I see.

Good thing there are tests.

:)

This is in clang-14 (for now): https://github.com/llvm/llvm-project/commit/f62d18ff140f67a8776a7a3c62a75645d8d540b5

If there is a need to silence anything, I think the correct approach is suggested there: cast one of the inputs to & to int.