Fix crash when a custom IMG model is restored while still in use

[Flashmyname]

Summary

SetStreamingInfo force-removes the model's RwObject, and the streamer stops tracking entities that still use it. LinkModel streams those entities out first; UnlinkModel and StreamDisable make the same call without it, so engineRestoreDFFImage - or destroying the IMG element - frees the RW object under live entities.

StreamDisable does call RestreamWorld(), but only after every SetStreamingInfo in the loop has already run, and not at all during session shutdown.

The guard moves into a file-local helper used by all three.

Motivation

The vehicle guard came from #4544 and the weapon one from #5146; neither covered the restore paths. CStreaming::RemoveModel at 0x4089A0 has no reference count - for a vehicle it calls CVehicleModelInfo::DeleteRwObject, which destroys the CVehicleStructure and the master clump regardless of who is using them.

Test plan

Custom IMG holding a banshee.dff, engineImageLinkDFF onto model 429, spawn a Banshee and get in, then engineRestoreDFFImage(429).

Before: 0xC0000005 at gta_sa.exe+0x124765, EAX=0 - CEntity::GetColModel returned null and the render path dereferenced it.

https://streamable.com/kc6ud6

After: the call returns true and the game keeps running.

https://streamable.com/fujmyi

Checklist

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