x11-selection: Plug MetaX11SelectionOutputStream leak

GNOME/mutter!991
4 jobs for master in 8 minutes and 27 seconds
latest
Status Job ID Name Coverage
  Build
passed #546065
build-mutter

00:04:52

passed #546066
build-without-native-backend-and-wayland

00:03:42

 
  Test
passed #546068
can-build-gnome-shell

00:01:32

passed #546067
test-mutter

00:03:35