xwayland: Remove pending stream reference when freeing

The EGLStream backend keeps a queue of pending streams for each Xwayland
window.

However, when this pending queue is freed, the corresponding private
data may not be cleared (typically if the pixmap for this window has
changed before the compositor finished attaching the consumer for the
window's pixmap's original eglstream), leading to a use-after-free and a
crash when trying to use that data as the window pixmap.

Make sure to clear the private data when the pending stream is freed.

Closes: #1055


Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Tested-by: Karol Szuster <karolsz9898@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>