Fix buffer length validation in ktc_GetToken and knfs

Fix buffer length validation in ktc_GetToken and knfs

The signed int tktLen is checked against a maximum size, then passed
as the unsigned size_t argument to memcpy.  So we need to make sure it
isn’t negative.

This doesn’t appear to be exploitable: tktLen comes from the kernel,
which should have previously validated the length within the SETTOK
pioctl.

This bug was found with STACK <http://css.csail.mit.edu/stack/>.

Change-Id: I781bd300cad3d725d3517e7f6ac9e6423c417087
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/11109
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>