ptserver: Tidy malloc handling in readpwd

Tidy up the malloc handling in readpwd, so that we don't leak memory
if the user specifies multiple -c arguments. Also avoid assuming that
free(NULL) will always work.

Change-Id: I95f3fe908572cb5be2d30345ccae0a2858622bd5
Reviewed-on: http://gerrit.openafs.org/9178
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>