heimdal: kadm5_c_get_principal() should check return code

kadm5_c_get_principal() should check the return code from
kadm5_ret_principal_ent(). As it doesn't it assumes success when
there is none and can lead to potential vulnerability. Fix this.

Reported by: rtm@csail.mit.edu

(cherry picked from commit f6d5d31cd5d5b409851b3bf82c41a477a643a486)