rx: fix abort on missing service

rx: fix abort on missing service

patchset 1fbe83f9aacfc36a9c426ba1fd18ad7c72869dc1 introduced
the possibility that a NULL connection pointer could be dereferenced.
Prevent it.

Reviewed-on: http://gerrit.openafs.org/7794
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 3ec1bdf9c74bd250d49065fa84475a5123461744)

Change-Id: I1e01005a34e5d373b6810052d94fac8d96f40e4f
Reviewed-on: http://gerrit.openafs.org/8371
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>