reflection: bounds-check the union type slot in the verifier

[evilgensec]

Problem

reflection::Verify is the runtime verifier for untrusted reflection-encoded FlatBuffers. When it verifies a union it reads the union type slot from the vtable without bounds-checking the voffset first, unlike the sibling value slot and every other field:

• VerifyVector (vector of unions) reads the type vector at vec_field.offset() - sizeof(voffset_t) via GetPointer with no VerifyField (4-byte read, up to ~64 KB past the buffer).
• VerifyObject (scalar union) reads the type byte at field_def->offset() - sizeof(voffset_t) via GetField<uint8_t> with no VerifyField (1-byte read).

A union value field sorts before its hidden type field, so the verifier reaches the unvalidated type slot first. A malformed buffer with a poisoned type voffset makes Verify read out of bounds. Both confirmed with ASAN.

Fix

Add the missing VerifyField guard in both branches, matching the existing value-slot and string-field checks. Add a reflection_test.cpp regression test that builds a schema plus a malformed buffer for the vector-of-unions and scalar-union cases and checks Verify rejects each instead of reading out of bounds.

[evilgensec]

@dbaileychess this is ready when you have a moment.

reflection::Verify reads the union type slot from the vtable without bounds-checking the voffset first: the type vector in VerifyVector and the type byte in VerifyObject, unlike the sibling value slot and every other field. A union value field sorts before its hidden type field, so the verifier reaches the unvalidated type slot first. A malformed buffer with a poisoned type voffset makes Verify read out of bounds (4 bytes, up to ~64 KB past the buffer, for the vector case; 1 byte for the scalar case). Both confirmed with ASAN.

The change adds the missing VerifyField guard in both branches, matching the existing value-slot and string checks, plus a reflection_test.cpp regression test covering the vector-of-unions and scalar-union cases. Full test suite passes under ASAN locally.