Hardening: add null guards and bounds checks to minireflect, hash functions, and flathash

[anagnorisis2peripeteia]

Summary

This PR hardens mini-reflection, hash computation, and the flathash tool against null pointer dereferences and integer underflow hazards:

1. minireflect.h:

   • InlineSize: Guard against type_table == nullptr and validate type_table->values != nullptr before indexing struct sizes.
   • IterateValue: Guard against visitor == nullptr and val == nullptr, safely forwarding null/missing values to visitor->Unknown(val) instead of performing unaligned reads. Guard type_table == nullptr in ET_SEQUENCE.
   • IterateObject: Add early exit when obj, type_table, or visitor is nullptr.
   • IterateFlatBuffer: Guard !buffer || !type_table || !callback and check root != nullptr before invoking IterateObject.
   • ToStringVisitor: Guard indent_level > 0 before decrementing in EndSequence() and EndVector() to prevent unsigned integer underflow. Guard str == nullptr in String() to avoid dereferencing null string pointers.
   • FlatBufferToString: Return empty string "" immediately when buffer or type_table is nullptr.

2. hash.h:

   • HashFnv1 & HashFnv1a: Guard !input and return 0 instead of immediately dereferencing *input.
   • FindHashFunction16, FindHashFunction32, FindHashFunction64: Guard !name before calling std::strcmp to prevent segmentation faults on null lookup keys.

3. flathash.cpp:

   • Guard argc > 0 && argv[0] and argv[1] in argument parsing.
   • In command line argument iteration loop, guard arg != nullptr before inspecting prefix characters and passing to hash functions.

4. Unit Tests:

   • Added MiniReflectHashSafetyTest() in tests/test.cpp verifying behavior with null buffers, null hash names, null strings in ToStringVisitor, and underflow resilience.
   • Verified clean compilation with -DFLATBUFFERS_STRICT_MODE=ON, flattests, and flatc_cpp_tests.py.