Fix crash if bonemerging to a pending model

[ficool2]

If an entity is bonemerging to a parent whose model has not been loaded yet, this will cause a server crash. This has been a top cause of crashing on a specific community server across various maps.

Specifically this is caused by CBaseAnimating::SetupBones not checking if the model pointer is valid when attempting to fetch the parent's bone cache. This PR introduces a check for validity.

In addition, all call sites to CBaseAnimating::GetBoneCache are required now to pass the model pointer. This is consistent with the client (see C_BaseAnimating::GetBoneCache), and also prevents inefficiency, as GetModelPtr() is expensive.