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)

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>
(cherry picked from commit 3c564444cf1c0bad25792b10edc158030e180369)

Change-Id: I8d0bb6ec6a39ad095959ede0252dc6f00777515e
Reviewed-on: http://gerrit.openafs.org/11051
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>