heimdal: Check return code from _hdb_fetch_kvno()

A malformed principal will cause kadmind to segfault. The PR
addresses only when chpass is requested but delete is also affected.
Fix both.

PR: 268002
Reported by: Robert Morris <rtm@lcs.mit.edu>

(cherry picked from commit 671f55828d038e17fb29fd115b88e7cdf53500c3)