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.

PR: 267884
Reported by: Robert Morris <rtm@lcs.mit.edu>

(cherry picked from commit d831a2fe480fe02126bd5b9aba5569c5e69f1034)