heimdal: Fix bus fault when zero-length request received

Zero length client requests result in a bus fault when attempting to
free malloc()ed pointers within the requests softc. Return an error
when the request is zero length.

PR: 268062
Reported by: Robert Morris <rtm@lcs.mit.edu>

(cherry picked from commit 6742ff42ab3b6e65239f975314060b1393e22d62)