rx: initialize memory before use

initialize sockaddr_in.sin_zero before using. cosmetic, might fix some
compiler warnings.

Change-Id: Ib9c4707373ca98742f6a5e28e60006499527fa38
Reviewed-on: http://gerrit.openafs.org/10816
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>