heimdal: Fix NULL dereference when mangled realm message

Fix a NULL dereference in _kadm5_s_init_context() when the client
sends a mangled realm message.

PR: 267912
Reported by: Robert Morris <rtm@lcs.mit.edu>

(cherry picked from commit 05bc50bdb1c1ddbbeb853ea4b184aced0eca9b3f)