Avoid handle a keysym if the compose state gets cancelled

This is the approach taken by libX11, and feels more consistent.

i3lock.c

@@ -339,7 +339,7 @@ static void handle_key_press(xcb_key_press_event_t *event) {
             break;
         case XKB_COMPOSE_CANCELLED:
             xkb_compose_state_reset(xkb_compose_state);
-            break;
+            return;
         }
     }