[ClutterMozHeadless] Don't send update when no shm
[ClutterMozHeadless] Don't send update when no shm

Don't send an update to a newly connected view if the shared memory
segment hasn't been initialised yet. Fixes a possible race on startup.

clutter-project/clutter-mozembed,clutter-project/clutter-mozembed,clutter-project/clutter-mozembed