pk-client: Fix runtime warning when backend does not support role

The daemon returns "op not supported by the backend" as part of the method
call response, but this response can be received after the "Destroy" signal,
which causes infinite wait for the transaction end and runtime warnings
about NULL error being passed to the g_task_return_error() on the client side.

This is a regression from commit PackageKit@44215db

lib/packagekit-glib2/pk-client.c

@@ -1616,11 +1616,11 @@ pk_client_signal_cb (GDBusProxy *proxy,
 	if (g_strcmp0 (signal_name, "Destroy") == 0) {
 		g_autoptr(GError) local_error = NULL;
 
-		if (state->waiting_for_finished)
+		if (state->waiting_for_finished) {
 			local_error = g_error_new_literal (PK_CLIENT_ERROR, PK_CLIENT_ERROR_FAILED,
 							   "PackageKit transaction disappeared");
-
-		pk_client_state_finish (state, g_steal_pointer (&local_error));
+			pk_client_state_finish (state, g_steal_pointer (&local_error));
+		}
 		return;
 	}
 }