Fix invalid JSON handling in KDC OTP module

If the OTP configuration for a principal contains invalid JSON, the
KDC OTP module calls k5_json_get_tid on a null pointer, causing the
KDC process to crash.  Fix this bug by checking the return value of
k5_json_decode in decode_config_json.

(cherry picked from commit dab1c234e15afdc64dfe776bdbc65bbc17d07e12)

https://github.com/krb5/krb5/commit/e52d893ec6701006bbf390856bf94bfe8da7c483
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: e52d893ec6701006bbf390856bf94bfe8da7c483
Branch: krb5-1.12
 src/plugins/preauth/otp/otp_state.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)