Hardening: add null guards, bounds checks, and optional safety to IDL generators and schema emitters

[anagnorisis2peripeteia]

Description

This PR defensively hardens IDL code generators and schema emitters against null dereferences and iterator/container bounds issues:

1. src/idl_gen_fbs.cpp:
   • Safeguards GenType against null struct_def or null defined_namespace.
   • Validates that used_proto_ids.size() >= 2 in HasTwiceUsedId and HasGapInProtoId before calling std::next(std::begin(...)) to eliminate undefined iterator advancing on small or empty vectors.
   • Guards against null FieldDef* entries in HasFieldWithId, HasNonPositiveFieldId, HasFieldIdFromReservedIds, ExtractProtobufIds, and MapProtoIdsToFieldsId.
   • In GenerateFBS: bounds-checks ns.from_table <= ns.components.size() before performing reverse component indexing and iterator subtractions; skips null namespace/enum/struct pointers; guards enum_def.defined_namespace and struct_def.defined_namespace; guards parser.opts.file_saver before invoking SaveFile.
2. src/idl_gen_binary.cpp:
   • Guards parser.opts.file_saver in GenerateBinary for both flexbuffers and standard flatbuffers.
   • Guards against null parser.root_struct_def_ before accessing its file member in BinaryMakeRule.
3. src/idl_gen_cpp.cpp:
   • Safeguards generate_bfbs_embed and generate against null parser_.opts.file_saver.
4. src/idl_gen_text.cpp:
   • Checks parser.opts.file_saver in GenTextFile before calling SaveFile, returning a failure string rather than dereferencing null.
5. tests/test.cpp:
   • Adds unit test IdlGeneratorsSafetyTest validating safe handling of null file_saver in GenTextFile and NewFBSCodeGenerator.

[anagnorisis2peripeteia]

Closing this pull request.

These checks guard states a correct parse does not produce, or they treat a missing file_saver as a successful generate. FlatBuffers does not put that check in the generators. A schema problem is a parser error. A caller mistake is FLATBUFFERS_ASSERT. An untrusted buffer is checked in the verifier.

See the C++ guide section "Access of untrusted buffers" and the review on #8990.