Fix vtable offset overflow when table contains >= 32,766 fields

[deezsecc]

Description

This PR fixes a critical validation gap in flatc (idl_parser.cpp) where computed vtable offsets could silently overflow and wrap around when a table defines 32,766 or more fields.

Details

• The vtable offset calculation formula is fixed_fields (4) + id * sizeof(voffset_t) (2).
• Since the offset is cast to voffset_t (uint16_t), once field_id reaches 32,766, the calculated offset reaches 65,536, which wraps to 0 and collides with the vtable vsize metadata slot. Field index 32,768 wraps to 4, colliding directly with VT_FIELD0 (offset 4).
• This allows a malformed buffer to bypass verification because the C++ Verifier validates layout using one field type, while the application reads using another (causing type confusion and OOB reads).

Fixes

• Auto-Incrementing Path (Parser::AddField): Added active validation returning a compiler error if the table fields count reaches 32,766 (which translates to a maximum offset of 65,534, the last value that safely fits in voffset_t).
• Explicit IDs Path (Parser::ParseDecl): Replaced a release-stripped FLATBUFFERS_ASSERT with an active Error() return check.

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