Hardening: add null guards, bounds checks, and return correctness to allocators, vector iterators, and stl emulation

[anagnorisis2peripeteia]

Description

This PR strengthens defensive engineering across FlatBuffers memory allocation, vector iterators, and STL emulation utilities:

1. include/flatbuffers/allocator.h:

   • Check if allocate(new_size) returns nullptr before performing downward copies and deallocating in reallocate_downward.
   • In memcpy_downward, verify pointers (new_p, old_p) are non-null and copy counts (in_use_back, in_use_front) are non-zero before invoking memcpy to avoid undefined behavior.

2. include/flatbuffers/default_allocator.h:

   • Guard against zero-byte allocation in allocate(size) (size ? new uint8_t[size] : nullptr).
   • Add null checks before calling array delete[] in deallocate, dealloc, and helper Deallocate.

3. include/flatbuffers/stl_emulation.h:

   • Fix missing return *this; in span::operator=(const span &other) which previously fell off the end of a value-returning function (undefined behavior).
   • Change pointer const data_; in span to pointer data_; so copy assignment can actually rebind the data pointer as specified.
   • Add mutable operator*, operator->, and mutable value() overloads to Optional matching standard std::optional semantics.

4. include/flatbuffers/vector.h:

   • In VectorIterator, avoid null pointer arithmetic (nullptr + offset) during construction and iterator offset operations.
   • In VectorIterator::operator-, check for element_stride != 0 before division to avoid division by zero.
   • In Vector::LookupByKey, return nullptr immediately if size() == 0 rather than executing binary search on empty ranges.
   • In Vector::KeyCompare, add null guards for key, data, and table before dereference.
   • Fix copy-paste issue in make_structs_span pointers overloads where make_span was invoked instead of make_structs_span.
   • In VectorCast, check for nullptr before reinterpret casting.

5. tests/test.cpp:

   • Added unit test StlEmulationAndVectorSafetyTest() validating Optional, span assignment, VectorIterator, VectorCast, and null-safe allocator behaviors.

All tests compile cleanly under -DFLATBUFFERS_STRICT_MODE=ON and pass with flattests and flatc_cpp_tests.py.