gs-updates-section: Fix a crash in _reboot_failed_cb()

If the widget is disposed between starting the async reboot call, and _reboot_failed_cb() being called, self is dereferenced after finalisation.

This fix is a guess: I can’t reproduce the crash myself.

Signed-off-by: Philip Withnall philip@tecnocode.co.uk

Fixes: #2325 (closed)

Closes #2325 (closed)