don't split process cycle at end

Keep in mind: While this is the correct thing to do,
it hides another another bug under the hood, related to
_global_port_buffer_offset in MidiPort::flush_buffers.
For debugging the latter issue, revert this.

libs/ardour/session_process.cc

@@ -421,7 +421,9 @@ Session::process_with_events (pframes_t nframes)
 				check_declick_out ();
 			}
 
-			_engine.split_cycle (this_nframes);
+			if (nframes > 0) {
+				_engine.split_cycle (this_nframes);
+			}
 
 			/* now handle this event and all others scheduled for the same time */