Playback doesn't work if Jack MIDI is used

[Andreas Schneider]

If MIDI is configured to use Jack, start playback plays back two notes
and then stops. Also, the note at the cursor is changed to the note of
the first staff while playing back. I can produce this behaviour e.g. by
creating a new score with two staffs, add a bunch of g's in the upper
staff and a bunch of c's in the lower staff, go to the beginning and
start playback with the playback control. This strange behaviour does
not occur if I change MIDI input to Portmidi; in the latter case
playback works as expected.

Andreas