wayland: check valid pending cairo surface

gdk_wayland_window_attach_image() is normally called from
gdk_window_end_paint() to notify the compositor of newly staged drawing.

If any of the drawing code inadvertently dispatches the wayland event
loop (for instance with a gdk_flush() call), then it's possible that by
the time gdk_window_end_paint() is called, the staged drawing is already
destroyed.

This commit bypasses the attach_image call in scenarios where the staged
drawing is prematurely dropped.

https://bugzilla.gnome.org/show_bug.cgi?id=773274

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