Freeze break: mutter: moving windows between workspaces

[Owen Taylor <otaylor redhat com>]

GNOME Shell currently crashes if you move a window between workspaces
when there is only one window open.

 https://bugzilla.gnome.org/show_bug.cgi?id=642957

As you can see from:

 https://bugzilla.redhat.com/show_bug.cgi?id=680207

this gets hit pretty often. I believe that the short patch from Dan to
work around the crash is very safe and would like to get it in.

- Owen