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>
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D37471