Release build with AVM_WARNINGS_ARE_ERRORS=ON fails on an unused variable in module.h #2448

Found while reviewing #2357; pre-existing (module.h is not changed by #2357). Configuring with -DCMAKE_BUILD_TYPE=Release -DAVM_WARNINGS_ARE_ERRORS=ON does not build, with gcc 15.2 and clang 21.1 alike:

src/libAtomVM/module.h:367:9: error: unused variable 'funs_count' [-Werror=unused-variable]

The error repeats for every translation unit that includes module.h. Debug builds, and the CI builds (which do not set CMAKE_BUILD_TYPE), are fine.

Cause: module_get_fun_arity_old_index_uniq (src/libAtomVM/module.h:364-376) reads funs_count only to check it in assert(fun_index < funs_count). A Release build defines NDEBUG, the assert disappears, and the variable becomes unused in a static inline function in a header.

Fix: mark it with UNUSED(funs_count) after the assert, or keep the read inside the assert (assert(fun_index < (int) READ_32_UNALIGNED(table_data + 8))), and add a Release + AVM_WARNINGS_ARE_ERRORS=ON configuration to CI so that this cannot come back.