Stop crashing on bad meshes like it's the end of the world

Pushing this on behalf of savis

src/EterGrnLib/ModelInstanceUpdate.cpp

@@ -177,6 +177,9 @@ void CGrannyModelInstance::UpdateWorldMatrices(const D3DXMATRIX* c_pWorldMatrix)
 
 		const CGrannyMesh * pMesh = m_pModel->GetMeshPointer(i);
 
+		if (!pMesh)
+			continue;
+
 		// WORK
 		int * boneIndices = __GetMeshBoneIndices(i);
 		// END_OF_WORK