xwayland: Plug leaks in xwl_present_sync_callback

xwl_present_window->sync_callback was leaked.

The event memory was leaked if the corresponding buffer had already been
released.
1 job for xwl_present_sync_callback-leaks in 1 minute and 56 seconds (queued for 2 minutes and 50 seconds)
Status Job ID Name Coverage
  Test
passed #33456
job

00:01:56