void *midi_buf;

    jack_midi_event_t in_event;

    jack_nframes_t event_index = 0;

    jack_nframes_t event_count;

    midi_buf = jack_port_get_buffer(_ports[MIDI], nframes);

    event_count = jack_midi_get_event_count(midi_buf);

    jack_midi_event_get(&in_event, midi_buf, event_index); // error here?

    std::cout << std::hex << static_cast<unsigned int> (*(in_event.buffer)) << std::endl;

    if (event_count)

    {
    }

What happens when event_index has a value of 0? It will attempt a read & dereference anyway.
Perhaps put the jack_midi_event_get() call into the if  statement.

Also, you should probably use this instead of if(event_count)

int event_num = 0;
while ( event_num < event_count )
{
   // do stuff
   event_num++;
}

Haven't tried the above code, but I think that might be it :)
HTH, -Harry