Fix alignment and de-click when monitoring changes while playing

Previously it was possible that
 * declick_out = true,
 * target_gain == 0, cur_gain != 0 (fade out active)
 * speed != 0, disk_samples_to_consume > 0.

So the disk-reader advanced the playback_sample, but since
declick_out is active, the read from the ringbuffer was not committed.

libs/ardour/disk_reader.cc

@@ -333,6 +333,7 @@ DiskReader::run (BufferSet& bufs, samplepos_t start_sample, samplepos_t end_samp
 		ms = MonitorState (ms | MonitoringDisk);
 		assert (result_required);
 		result_required = true;
+		disk_samples_to_consume = 0; // non-committing read
 	} else {
 		_declick_offs = 0;
 	}