readgroup: Make sure user supplies a groupfile

If the user supplies enough command line arguments, but doesn't
provide a groupfile, then we can end up trying to open whatever
garbage is on the stack.

Once we've finished parsing the command line arguments, make sure
that a filename was supplied.

Caught by coverity (#986008)

Change-Id: Iaea60027cf4bd81f9af04e995ce5c395275be833
Reviewed-on: http://gerrit.openafs.org/9440
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>