Don't allocate then discard notes on note off.

Don't allocate then discard notes on note off.

Silly to make a junk Note just to pass to append_note_off_unlocked, which just
uses the fields that are on the MIDIEvent anyway then throws it away.

Also explicitly dispatch to append_note_off_unlocked in the caller for note ons
with velocity 0 rather than make append_note_on_unlocked deal with it.