The gbm for the output might be null for the pageflip in the case that
the output has been disconnected. The gbm might be set to null by
wlr_drm_output_cleanup() in this case.
If the output is cleaned up before the pageflip, then a double free
will crash the compositor on the call to gbm_surface_release_buffer()
in the pageflip handler. The outputs buffer object bo[1] will point to
invalid memory.