Fix handling of invalid CAMMAC service verifier

In extract_cammacs(), avoid a null dereference if the CAMMAC service
verifier is invalid or the CAMMAC is empty.

https://github.com/krb5/krb5/commit/8451ff6ed57361de585a35f35a39c54dc48172c7
Author: Jeffrey Arbuckle <jeffa.lans@gmail.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 8451ff6ed57361de585a35f35a39c54dc48172c7
Branch: master
 src/lib/krb5/krb/authdata.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)