rx: fix call refcount leak in error case

rx: fix call refcount leak in error case

The recent event handling normalization in commit
304d758983b499dc568d6ca57b6e92df24b69de8 had event handlers switch
to dropping their reference on the associated connection/call just
before return.  An early return case was missed in the conversion,
leading to a refcount leak in an error case.

Reviewed-on: https://gerrit.openafs.org/12781
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 66b74e78ba5fea6a8236dcd3b8b46e1dfa6a0ac7)

Change-Id: I532c49b2ef6ec95dd26a99c02e12ea53348f9690
Reviewed-on: https://gerrit.openafs.org/12783
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>