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>
MFC after: 3 days