traceback object init - set it to empty only if null

py/objexcept.c

@@ -558,11 +558,10 @@ void mp_obj_exception_add_traceback(mp_obj_t self_in, qstr file, size_t line, qs
         if (self->traceback == NULL) {
             self->traceback = &MP_STATE_VM(mp_emergency_traceback_obj);
         }
+        // populate traceback object
+        *self->traceback = mp_const_empty_traceback_obj;
     }
 
-    // populate traceback object
-    *self->traceback = mp_const_empty_traceback_obj;
-
     // append the provided traceback info to traceback data
     // if memory allocation fails (eg because gc is locked), just return
     if (self->traceback->data == NULL) {