Prior to the DisplayConfig merge, we would set _NET_WM_FULLSCREEN_MONITORS
to (unsigned)-1 when unset. After that, we would have invalid
reads inside meta_screen_monitor_index_to_xinerama_index() (called
with -1).
The way I read the specification, the proper way to indicate
that the window is back to fullscreen on all monitors is to
remove the property, so do that.

Also, add an assertion that meta_screne_monitor_index_to_xinerama_index()
is doing the right thing.