lock mixer while adding BB-track (finally closes #2014486)

git-svn-id: https://lmms.svn.sf.net/svnroot/lmms/trunk/lmms@1344 0778d3d1-df1d-0410-868b-ea421aaaa00d

src/core/song.cpp

@@ -641,9 +641,11 @@ void song::removeBar( void )
 
 void song::addBBTrack( void )
 {
+	engine::getMixer()->lock();
 	track * t = track::create( track::BBTrack, this );
 	engine::getBBTrackContainer()->setCurrentBB(
 						bbTrack::numOfBBTrack( t ) );
+	engine::getMixer()->unlock();
 }