volser/vol_split.c: do not dereference known-NULL pointer

Avoid dereferencing a pointer that we just checked is NULL when printing
an error message.

Reviewed-on: http://gerrit.openafs.org/8878
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 5c5515a5c3b34f9a115b29378e123c779c138f51)

Change-Id: I73de73f8cd9be9d106285f45e86e030909293521
Reviewed-on: http://gerrit.openafs.org/9512
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>