bug:
- add midi track
- add additional midi port
- remove additional midi port
- press play
-> crash
because..
ARDOUR::MidiTrack::roll's route_buffer still has 2 midi-channels. This is
because processor_max_streams was not updated (no plugins added/removed)
fill_buffers_with_input() eventually asserts when trying to get the 2nd
(nonexistant) midi port via ARDOUR::PortSet::nth_midi_port() in
ARDOUR::PortSet::port().
Robin Gareus
35703cb3fc