do not crash when loading settings where no file is set

git-svn-id: https://lmms.svn.sf.net/svnroot/lmms/trunk/lmms@746 0778d3d1-df1d-0410-868b-ea421aaaa00d

plugins/patman/patman.cpp

@@ -170,6 +170,12 @@ void patmanInstrument::deleteNotePluginData( notePlayHandle * _n )
 
 void patmanInstrument::setFile( const QString & _patch_file, bool _rename )
 {
+	if( _patch_file.size() <= 0 )
+	{
+		m_patchFile = QString::null;
+		return;
+	}
+
 	// is current instrument-track-name equal to previous-filename??
 	if( _rename &&
 		( getInstrumentTrack()->name() ==