Hardening: add null guards, bounds checks, and buffer underflow safety to flexbuffers

[anagnorisis2peripeteia]

Summary

This PR hardens include/flatbuffers/flexbuffers.h against null pointer dereferences, buffer underflows, and out-of-bounds access across the reader, builder, and verifier APIs:

1. GetRoot Underflow & Truncation Protection:

   • GetRoot(buffer, size) requires at least 3 bytes (size >= 3) and validates size >= static_cast<size_t>(byte_width) + 2 before decrementing pointers. Previously, passing a null or zero-length buffer caused pointer arithmetic before the buffer (*--end), triggering heap/stack buffer underflow.

2. Null Guards in Scalar & Indirect Decoders:

   • ReadSizedScalar, ReadInt64, ReadUInt64, ReadDouble guard against data == nullptr and return safe defaults (0 / 0.0).
   • Indirect checks !offset || byte_width == 0 and returns nullptr.

3. Null-safe Reference, Vector, and Map Access:

   • Sized::read_size() guards !data_ || byte_width_ == 0.
   • Reference accessor methods (AsInt64, AsDouble, AsBool, AsKey, AsString, AsBlob, AsVector, etc.) guard against !data_ and validate indirect offsets before dereferencing.
   • Reference::Mutate* checks data_ != nullptr, dest != nullptr, and input pointers.
   • Vector::operator[], TypedVector::operator[], FixedTypedVector::operator[] guard !data_.
   • Map::operator[](const char* key) guards against null key and null keys.data_, preventing crashes in std::bsearch and KeyCompare.

4. Builder and Verifier Input Validation:

   • Builder::Key and Builder::String safely handle null string pointers.
   • Builder::WriteBytes and Builder::CreateAlignedBlob handle null data.
   • Builder::EndVector and Builder::EndMap guard against start > stack_.size().
   • Verifier::VerifyOffset and Verifier::VerifyBuffer validate buf_ != nullptr before pointer comparisons and buffer size checks.

5. Unit Tests:

   • Added FlexBuffersSafetyTest() in tests/flexbuffers_test.cpp verifying behavior with null buffers, truncated inputs, null map keys, and empty builder calls.
   • Verified clean compilation with -DFLATBUFFERS_STRICT_MODE=ON, flattests, and flatc_cpp_tests.py.