Fix JACK audio disconnection of device inputs

[dagargo]

In 887f18e, only writing to the j2o ring buffer was performed if there were inputs connected.

This works well but if we later disconnect the inputs, the OB end keeps reading and resampling from the queue, what creates a noisy and undesirable output.

[dagargo]

Fixed in 75c9970.

Instead of checking the status of the input ports in every loop, we use a member of the overbridge struct to indicate this.

Then, this variable is used to bypass both ends of the j2o audio side.

o2j audio is always available and so is MIDI both ways.