closes bpo-34471: _datetime: Add missing NULL check to tzinfo_from_isoformat_results. (GH-8869)

Reported by Svace static analyzer.

Modules/_datetimemodule.c

@@ -1290,8 +1290,11 @@ tzinfo_from_isoformat_results(int rv, int tzoffset, int tz_useconds) {
         }
 
         PyObject *delta = new_delta(0, tzoffset, tz_useconds, 1);
+        if (delta == NULL) {
+            return NULL;
+        }
         tzinfo = new_timezone(delta, NULL);
-        Py_XDECREF(delta);
+        Py_DECREF(delta);
     } else {
         tzinfo = Py_None;
         Py_INCREF(Py_None);