xwayland/present: Destroy sync_callback in xwl_present_cleanup

Merged Michel Dänzer requested to merge daenzer/xserver:xwl_present_cleanup-sync_callback into master

xwl_present_cleanup frees the struct xwl_present_window memory, so if there's a pending callback, we have to destroy it to prevent use-after-free in xwl_present_sync_callback.

Should fix issue #645 (closed).

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