MER: [compositor] Avoid potential crash in compositor.
MER: [compositor] Avoid potential crash in compositor.

If we called disown on a buffer after it got destroyed we would
touch the m_buffer handle which was then invalid which would
crash.

For upstream this functionality is implemented in
https://codereview.qt-project.org/#change,81757

locusf/qtwayland-1,locusf/qtwayland-1,locusf/qtwayland-1,locusf/qtwayland-1