Work around a bug in QueryRow in the simple query path

In f8ffc32df8b9c5fd7d5ca1ac83 I fixed this for the extended query path
where the problem was much more prevalent, but issue #266 pointed out a
case where the simple query path is vulnerable to the same bug, which I
had missed in my original fix.

While at it, also add some more sanity checking to the conn.saveMessage*
handling.