start out this branch always with filename NULL

Python/_warnings.c

@@ -514,7 +514,7 @@ setup_context(Py_ssize_t stack_level, PyObject **filename, int *lineno,
     }
     else {
         const char *module_str = PyString_AsString(*module);
-        Py_XDECREF(*filename);
+        *filename = NULL;
         if (module_str && strcmp(module_str, "__main__") == 0) {
             PyObject *argv = PySys_GetObject("argv");
             if (argv != NULL && PyList_Size(argv) > 0) {