This error was just logged but not raised. Do as the code comment said
and raise a pipe error at that moment, and for subsequent operations
on the output stream (although none besides close() should be expected
after propagating the error properly).
Related: https://gitlab.gnome.org/GNOME/mutter/issues/1065
MetaX11SelectionOutputStream was storing copies of strings only to use
them in init and then free them in finalize. This was also causing a
small leak, because one of these strings was not freed. Instead of doing
that just don't create these unnecessary copies in the first place.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1001
The streams were only detached from MetaX11Display (and its event handling)
on completion. This is too much to expect, and those might be in some
circumstances replaced while operating.
Make those streams detach themselves on dispose(), so we don't trip into
freed memory later on when trying to dispatch unrelated X11 selection events.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/869
