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)

Change-Id: I997fd1812c45e730db39497a27a5e168f102fee5
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>