crasher when no alsa midi in/out ports are set

1. Use also midi api
2. Set in/out ports to zero
3. Create a patch with [nbx]---[midiout]
4. Click the "nbx" and drag it to send a stream of values

This will crash Purr Data.

It appears that snd_seq_event_output_direct in s_midi_alsa.c can get called with a midi_handle argument still set to NULL. I'm assuming that happens because our ports are still zero, but there may be other corruption happening in the upstream dataflow related to #555