Fix PyUpb_Message_MergeInternal segfault (#1338)

when `PyUpb_Message_MergeFromString` returns `NULL`, currently
`PyUpb_Message_MergeInternal` will call `Py_DECREF` on `NULL`
which results in a segmentation fault.

This patch switches to `Py_XDECREF` to fix the segfault.

python/message.c

@@ -1208,7 +1208,7 @@ static PyObject* PyUpb_Message_MergeInternal(PyObject* self, PyObject* arg,
   if (!serialized) return NULL;
   PyObject* ret = PyUpb_Message_MergeFromString(self, serialized);
   Py_DECREF(serialized);
-  Py_DECREF(ret);
+  Py_XDECREF(ret);
   Py_RETURN_NONE;
 }