OptimizeGraph: Fix possible null pointer dereference

code/OptimizeGraph.cpp

@@ -233,11 +233,13 @@ void OptimizeGraphProcess::CollectNewChildren(aiNode* nd, std::list<aiNode*>& no
 
     nd->mNumChildren = static_cast<unsigned int>(child_nodes.size());
 
+    if (nd->mChildren) {
     aiNode** tmp = nd->mChildren;
     for (std::list<aiNode*>::iterator it = child_nodes.begin(); it != child_nodes.end(); ++it) {
         aiNode* node = *tmp++ = *it;
         node->mParent = nd;
     }
+    }
 
     nodes_out += static_cast<unsigned int>(child_nodes.size());
 }