Fix MIDI Capture alignment

When recording audio, simply not writing to the ringbuffer
offsets the recording accordingly.
When recording MIDI, absolute timestamps are used, so the recording
has to be offset by the accumulated difference.

Previously this went unnoticed because tests using the Dummy
backend the accumulated offset never exceeded 1 cycle.

libs/ardour/disk_writer.cc

@@ -484,6 +484,8 @@ DiskWriter::run (BufferSet& bufs, samplepos_t start_sample, samplepos_t end_samp
 		 */
 		if (rec_nframes) {
 			_accumulated_capture_offset += rec_offset;
+		} else {
+			_accumulated_capture_offset += nframes;
 		}
 
 	}