Improve JACK midi robustness

[jpcima]

In this work, I try to make buffered Jack output more robust, as I discovered a few annoyances.
It enables some recovery from failures.

• When a message is written to the ring buffer by sendMessage, the outcome is not checked.
  If a message too big for the remaining buffer is written, and then passes a short message which succeeds, the validity of the message stream following is potentially broken.
  (because the size was communicated onto the size buffer, despite that message data did not go through, or just partial)

• The call jack_midi_event_reserve can fail with NULL.
  This case should be checked to prevent crashing. In this case, I will discard the buffered event, also permitting the stream to get back on its feet.

- If the MIDI processor cannot reserve the event memory, handle the NULL case
to avoid crashing, and skip data.
- Before writing the ringbuffer, verify it has available size, waiting if needed.
- Implement as a single ringbuffer, to improve memory efficiency.

[radarsat1]

Tested with c/qmidiin and midiout