Backport fixes from issue240 to gnome-3-28

Merged Olivier Fourdan requested to merge ofourdan/mutter:backport-issue240 into gnome-3-28

issue #240 (closed) was reported against mutter-3.28.3, we'd need to backport those fixes in gnome-3-28.

Also, those fixes may also need by commit 398f03bd which was cherry-picked a while ago, without this, commit 398f03bd may crash.

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