rxgen: Fix NULL pointer dereference

Avoid a NULL pointer dereference if strchr doesn't find any occurence
of '*' in the string. Whilst we handle the not found case when inserting
a mid string terminator, we don't handle it when restoring the string to
its previous value.

Reviewed-on: http://gerrit.openafs.org/9158
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 0d28f8c0cec91d68023795e6ba9c769d60869f9e)

Change-Id: I6eb3ec5647153e588c5a80025da2bf2c921469b3
Reviewed-on: http://gerrit.openafs.org/9525
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>