Fix NULL ptr dereferencing in local_timezone(). nameo can be NULL

CID 1040362 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)
var_deref_op: Dereferencing null pointer _py_decref_tmp.

Modules/_datetimemodule.c

@@ -4749,7 +4749,7 @@ local_timezone(PyDateTime_DateTime *utc_time)
             goto error;
     }
     result = new_timezone(delta, nameo);
-    Py_DECREF(nameo);
+    Py_XDECREF(nameo);
   error:
     Py_DECREF(delta);
     return result;