Support larger RADIUS attributes in libkrad

In kr_attrset_decode(), explicitly treat the length byte as unsigned.
Otherwise attributes longer than 125 characters will be rejected with
EBADMSG.

Add a 253-character-long NAS-Identifier attribute to the tests to make
sure that attributes with the maximal number of characters are working
as expected.

[ghudson@mit.edu: used uint8_t cast per current practices; edited
commit message]

https://github.com/krb5/krb5/commit/f35077bfc570205092eca2a9d44e50ce265622f4
Author: Sumit Bose <sbose@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: f35077bfc570205092eca2a9d44e50ce265622f4
Branch: master
 src/lib/krad/attrset.c  |    2 +-
 src/lib/krad/t_packet.c |   13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletions(-)