MidiInputPort and MidiOutputPort fix

[stijnvanbeek]

Fixes (backwards compatibility?) problem with ctors in MidiInputPort and MidiOutputPort that causes the pointer to the MidiService not to be set.

[cklosters]

This bug was introduced here (Make support for accessing midi ports using RtMidi optional in cmake), midiservice.cpp line 41 -> removal of object creators.

[cklosters]

I'm also missing an assert every time the service if de referenced -> handle should be set on creation or initialization, if not it is a coding error and should be asserted at runtime. Please take that into account for future reference, it helps others spot issues and debug your code.