visit_decref(): Added another assert.

Modules/gcmodule.c

@@ -224,6 +224,7 @@ visit_decref(PyObject *op, void *data)
 		 * generation being collected, which can be recognized
 		 * because only they have positive gc_refs.
 		 */
+		assert(gc->gc.gc_refs != 0); /* else refcount was too small */
 		if (gc->gc.gc_refs > 0)
 			gc->gc.gc_refs--;
 	}