Fix assert failure when cycle_end is called by AudioEngine::set_session before the buffer is set up.

git-svn-id: svn://localhost/ardour2/branches/3.0@9143 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/ardour/audio_port.cc

@@ -59,7 +59,9 @@ AudioPort::cycle_end (pframes_t nframes)
                 /* we can't use nframes here because the current buffer capacity may 
                    be shorter than the full buffer size if we split the cycle.
                 */
-		_buffer->silence (_buffer->capacity());
+		if (_buffer->capacity () > 0) {
+			_buffer->silence (_buffer->capacity());
+		}
 	}
 }