bos: don't dereference a null pointer when printing an error message

bos: don't dereference a null pointer when printing an error message

The parameter we are interested in is at MRAFS_OFFSET + 17, not
MRAFS_OFFSET + 13.

Found-by: clang static analyzer
Reviewed-on: http://gerrit.openafs.org/5178
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit bf4b2fd3e26dcf2a0db704815e05d77a558d38c6)

Change-Id: I92db9e5ec10054cef4421ab1b2fd0f60baaf07ca
Reviewed-on: http://gerrit.openafs.org/5222
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>