Verify, not Assert, for the (util) global mutex

We don't want turning off debugging to force open this lock.

Change-Id: Ia8dd689c506e42a4bf87c83af23d7457df12f8b9
Reviewed-on: http://gerrit.openafs.org/10092
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>