viced: fix log message for MapName_r

Do not log a code of zero if the name plus instance length exceeds
the max pr name buffer.

Reviewed-on: http://gerrit.openafs.org/6861
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit ef6ae1e7eda56e1d6ff7b9ade25b290c855e5780)

Change-Id: If644670ed522eaba575e7246084f106231649991
Reviewed-on: http://gerrit.openafs.org/9474
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>