Harden reflection verifier against malformed schemas

[M0nd0R]

Summary

• reject zero or negative reflected struct sizes before struct/vector verification uses them
• reject vector-of-union fields whose value vector is present but the companion type vector is missing
• add C++ reflection regression tests for both malformed-schema cases

Validation

• g++ -std=c++11 -Iinclude /tmp/flatbuffers_reflection_union_poc.cpp src/reflection.cpp src/util.cpp -o /tmp/flatbuffers_reflection_union_poc && /tmp/flatbuffers_reflection_union_poc returned Verify returned 0 after the fix; before the fix it crashed with SIGSEGV at flatbuffers::Vector<unsigned char>::size()
• g++ -std=c++11 -Iinclude /tmp/flatbuffers_reflection_struct_size_poc.cpp src/reflection.cpp src/util.cpp -o /tmp/flatbuffers_reflection_struct_size_poc && /tmp/flatbuffers_reflection_struct_size_poc returned Verify returned 0 after the fix; before the fix it crashed with SIGFPE in VerifierTemplate::VerifyVectorOrString()
• cmake -S . -B /tmp/flatbuffers-build -DCMAKE_BUILD_TYPE=Debug -DFLATBUFFERS_BUILD_TESTS=ON -DFLATBUFFERS_BUILD_FLATC=ON -DFLATBUFFERS_BUILD_FLATLIB=ON -DFLATBUFFERS_BUILD_FLATHASH=OFF -DFLATBUFFERS_BUILD_GRPCTEST=OFF
• cmake --build /tmp/flatbuffers-build --target flattests -j2
• /tmp/flatbuffers-build/flattests
• ctest --test-dir /tmp/flatbuffers-build --output-on-failure -R flattests
• git diff --check

clang-format was not available in this environment, so I validated formatting through compile/test plus git diff --check.