do not decref value borrowed from list (closes python#27774)

Author: benjaminp

Misc/NEWS

@@ -29,6 +29,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #27774: Fix possible Py_DECREF on unowned object in _sre.
+
 - Issue #27760: Fix possible integer overflow in binascii.b2a_qp.
 
 - Issue #27758: Fix possible integer overflow in the _csv module for large record

Modules/_sre.c

@@ -3401,10 +3401,8 @@ match_groupdict(MatchObject* self, PyObject* args, PyObject* kw)
         if (!key)
             goto failed;
         value = match_getslice(self, key, def);
-        if (!value) {
-            Py_DECREF(key);
+        if (!value)
             goto failed;
-        }
         status = PyDict_SetItem(result, key, value);
         Py_DECREF(value);
         if (status < 0)