Fix crash when assigning invalid model TXD IDs

[HeresHavi]

Summary

Added TXD slot validation to engineSetModelTXDID and made the pool availability check bounds safe.

Invalid or unallocated TXD IDs are now rejected before the model or its reference counts are changed.

Motivation

engineSetModelTXDID checked the model ID but accepted any unsigned 16-bit TXD ID. This allowed an unavailable slot to remain stored on a loaded model. A later operation such as engineGetModelTextureNames could pass that ID into GTA's unchecked TXD lookup and crash the client.

For example, a resource might cache a TXD ID so it can update a player model later. If that slot was released during a resource restart, reusing the stale ID could leave the model pointing at missing TXD data. The next texture query could then crash the player.

Valid IDs returned by engineRequestTXD are still accepted.

Crash Stack

Exception: Access violation
Module: gta_sa.exe
Offset: 0x003F3737

CRenderWareSA::GetModelTextureNames (CRenderWareSA.cpp:786)
CLuaEngineDefs::EngineGetModelTextureNames (CLuaEngineDefs.cpp:1457)

Test plan

Runtime

• Before Fix: TXD ID 65535 was accepted for a loaded model, and querying its texture names caused an access violation in GTA's TXD lookup.
• After Fix: The invalid ID was rejected, the original TXD remained assigned, and texture names could still be queried safely.
• Valid Case: The model's existing TXD and a new slot returned by engineRequestTXD were both accepted.
• Stale Case: An allocated slot was released through resource teardown, and its old in-range ID was rejected without changing the model.

Builds and Tests

• Debug | Win32: Solution build passed, 304 client tests passed.
• Release | Win32: Solution build passed, 304 client tests passed.
• Ran clang-format.

Checklist

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