security/heimdal*: The version string must always contain a terminating NUL
Should the sender send a string without a terminating NUL, ensure that
the NUL terminates the string regardless.
And while at it only process the version string when bytes are returned.
Reported by: Robert Morris <firstname.lastname@example.org>
(cherry picked from commit d831a2fe480fe02126bd5b9aba5569c5e69f1034)