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.

Change-Id: Ie3d0bc9474fdbc09be9c753f4d0192c8cca68351
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>