login-common: client_unref() - always set client pointer to NULL

This is the common coding practise elsewhere as well.

src/login-common/client-common.c

@@ -326,12 +326,12 @@ bool client_unref(struct client **_client)
 {
 	struct client *client = *_client;
 
+	*_client = NULL;
+
 	i_assert(client->refcount > 0);
 	if (--client->refcount > 0)
 		return TRUE;
 
-	*_client = NULL;
-
 	i_assert(client->destroyed);
 	i_assert(client->login_proxy == NULL);