heimdal: Properly ix 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.

This properly fixes PR/268062 without regressions.

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

(cherry picked from commit 3deefb0d147d71047a13ec2328b1b721da2ce256)