night-ligth-dialog: Avoid dereferencing invalid pointer

dialog_got_proxy_cb and dialog_got_proxy_props_cb may be called after the
instance of CcNightLightDialog has been disposed. Make sure 'self' pointer is
not dereferenced if not valid.

Fixes: GNOME/gnome-control-center#86
2 jobs for fix-issue-86 in 11 minutes and 24 seconds (queued for 2 seconds)
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passed build #31595

00:05:51

 
  Test
passed test #31596

00:05:32