kauth: Fix overflow when writing ticket file

krb_write_ticket_file uses a fixed length buffer to store the name
of the ticket file, but copies into this from an environment variable.

Remove the fixed length buffer, and use a mixture of the variable
itself, and dynamically allocated strings.

Caught by coverity (#985909)

Reviewed-on: http://gerrit.openafs.org/9294
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 a332ce9d0e87fab55f3d286690026fe075f624dd)

Change-Id: Idcf442323b13cc4daa893917ede6492616ba1aeb
Reviewed-on: http://gerrit.openafs.org/11021
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>