heimdal: Fix NULL deref

heimdal: Fix NULL deref

A flawed logical condition allows a malicious actor to remotely
trigger a NULL pointer dereference using a crafted negTokenInit
token.

Upstream notes:

    Reported to Heimdal by Michał Kępień <michal@isc.org>.

    From the report:

    Acknowledgement
    ---------------

    This flaw was found while working on addressing ZDI-CAN-12302: ISC BIND
    TKEY Query Heap-based Buffer Overflow Remote Code Execution
    Vulnerability, which was reported to ISC by Trend Micro's Zero Day

Security:	CVE-2022-3116
Obtained from:	upstream 7a19658c1
MFC after:	1 week