readpwd: Make sure user supplies a passwdfile

If the user supplies enough command line arguments, but doesn't
provide a passwdfile, 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 (#986009)

Change-Id: Ice2fca16458a90d73ae6b5fadb0efa22ed0b185a
Reviewed-on: http://gerrit.openafs.org/9441
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>