replace assignment with assertion

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -7526,7 +7526,7 @@ upload_texture(CLP(TextureContext) *gtc, bool force) {
 
     GraphicsEngine *engine = get_engine();
     nassertr(engine != (GraphicsEngine *)NULL, false);
-    engine = GraphicsEngine::get_global_ptr();  // temp hack
+    nassertr(engine == GraphicsEngine::get_global_ptr(), false);  // temp hack
     engine->texture_uploaded(tex);
     gtc->mark_loaded();