Responsibility for calling fluid_sequencer_unregister_client() in case of FLUID_SEQ_UNREGISTERING events has been moved to fluid_sequencer_send_now(). In other words, a FLUID_SEQ_UNREGISTERING event now really unregisters the client, no matter how the client's callback function looks like.
Avoids leaking the sequencer clients if implementations do not unregister them explicitly.
Also fixes another memory leak if fluid_sequencer_register_fluidsynth() clients were unregistered with fluid_sequencer_unregister_client() rather than by sending an unregistering event.
