ubik: Don't leak memory if mutex init fails

If we can't initialise the ubik_client mutex, don't leak the
structure when returning an error.

Caught by clang-analyzer

Change-Id: Ib36d2ddf585a31b628e56dce1f33ee859fc835bc
Reviewed-on: http://gerrit.openafs.org/9182
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>