rxgen: Fix memory leak

Don't leak the definition structure when we hit the end of the file

Change-Id: Ief44bb3bdff274167760fda5bcc16903ef8c11ce
Reviewed-on: http://gerrit.openafs.org/9159
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>