bugfix: add null check on output gbm on pageflip

Created by: acrisci

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.