window-actor: Guard against %NULL frame mask

Creating a new cogl texture may fail, in which case the intent to
free it will crash. While something is clearly wrong (insanely
large window, oom, ...), crashing the WM is harsh and we should
try to avoid it if at all possible, so carry on.

https://bugzilla.gnome.org/show_bug.cgi?id=722266
keybindings: Make move-to-monitor-* bindings known to g-c-c

Commit 0fe5c4f95774b85 implemented new keybindings to move windows
between monitors, but did not add it to the appropriate keybinding
file.

https://bugzilla.gnome.org/show_bug.cgi?id=725338