Simplify PyDict_SetDefaultRef result check in compile.c

Author: colesbury

Python/compile.c

@@ -983,12 +983,9 @@ merge_consts_recursive(PyObject *const_cache, PyObject *o)
 
     PyObject *t;
     int res = PyDict_SetDefaultRef(const_cache, key, key, &t);
-    if (res < 0) {
-        Py_DECREF(key);
-        return NULL;
-    }
-    if (res == 1) {
-        // o was already registered in const_cache.  Just use it.
+    if (res != 0) {
+        // o was not inserted into const_cache. t is either the existing value
+        // or NULL (on error).
         Py_DECREF(key);
         return t;
     }