test/interactive-x11: handle NULL from xcb_wait_for_event

Can happen in cases like:
- There was an error between the error check and the call.
- The internal poll() fails.

Signed-off-by: Ran Benita <ran234@gmail.com>

test/interactive-x11.c

@@ -276,6 +276,10 @@ loop(xcb_connection_t *conn, struct keyboard *kbd)
         }
 
         event = xcb_wait_for_event(conn);
+        if (!event) {
+            continue;
+        }
+
         process_event(event, kbd);
         free(event);
     }