compositor: Check buffer validity before attaching to renderer

Recent changes in c08a6ff8 defer the buffer attachment to the render loop, however dmabuf buffers can be destroyed (fds closed, etc) before we get there.

At that point it's too late to do anything as there's nothing left of the buffer except the empty structure waiting to be freed when the refcount hits zero.

Just don't bother attaching the dead buffer, and use the last attached buffer instead.

Signed-off-by: Derek Foreman derek.foreman@collabora.com