[LIBMIDI]remove hack with lastEvent = -1 (though it's an unsigned number...)

[allan-simon]

in libmidi , the MidiTrack::reset method set lastEvent to -1 though it's an unsigned (so as you guess we got an integer overflow)

it does not cause any bug , because we always use it with +1 , but well that's a hack...

[allan-simon]

I've started to mark the place where to lastEvent with a comment containing "#3" to make it easier to track