waylandsink: prevent frame callback being released twice

Randy Li requested to merge ayaka/gst-plugins-bad:dev_wayland2 into master

For those using context from the application which would be the embedded video case, if the frame callback is entering at the same time as window is finalizing, a wayland proxy object would be destroyed twice, leading the refcout less than zero in the second time, it can throw an abort() in wayland.

For those top window case, which as a directly connection to the compositor, they can stop the message queue then the frame callback won't happen at the same time as the window is finalizing. It doesn't think it would bother them about this.

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