SOLARIS: Do not release NULL root vp on unmount

SOLARIS: Do not release NULL root vp on unmount

When we unmount, and afs_globalVp is NULL (e.g. because root.afs was
unavailable when the client was started), we will panic the machine if
we try to release it. So, if afs_globalVp is NULL when we hit our
unmount handler, don't touch it.

Reported by Andy Cobaugh.

Change-Id: I4e5869237e24da320afc2b7edaf8edca0ab3e4e9
Reviewed-on: http://gerrit.openafs.org/5117
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>