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.

Change-Id: Ibc89883977e7044cab2a844d97b7f0f7d236f4ea
Reviewed-on: http://gerrit.openafs.org/8878
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>