Gnome Shell freeze break exception

[drago01 <drago01 gmail com>]

Hi,

I'd like to push the patch from
https://bugzilla.gnome.org/show_bug.cgi?id=696447 for 3.8.0
it fixes a regression that sneaked in while porting the hot corner to
pressure barriers.

The patch is very small (one liner) and fixes a regression in the
fallback path (when no barriers are present,
which happens when running on older X).