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.

ticket: 7912 (new)
target_version: 1.12.2
tags: pullup