Right now the XSync based idle monitoring code, will fetch all active
watches into a list, and then call their watch callbacks one by one
as necessary. If one watch callback invalidates another watch, the
list will contain free'd memory.
This commit makes sure to consult the hash table after ever call
of a watch callback, to ensure mutter never looks at freed memory.
Fixes crash reported on IRC by Laine Stump with his synergy setup.
https://bugzilla.gnome.org/show_bug.cgi?id=760330
