Prevent a crash when loading a group with no additions in SongManager::LoadSongDir

When reloading and only loading additions, ensure we do not delete/readd groups that were already loaded.

src/SongManager.cpp

@@ -436,10 +436,14 @@ void SongManager::LoadSongDir( RString sDir, LoadingWindow *ld, bool onlyAdditio
 		RString group_base_name= Basename(sGroupDirName);
 		Group* group = new Group(sDir, sGroupDirName);
 		
+		// We need to keep track of previously loaded groups so we don't delete them if we're only loading additions
+		bool groupAlreadyLoaded = false;
 		// Add the group to the group mapping
 		if (m_mapNameToGroup.find(sGroupDirName) == m_mapNameToGroup.end())
 		{
 			m_mapNameToGroup[sGroupDirName] = group;
+		} else {
+			groupAlreadyLoaded = true;
 		}
 
 		for( unsigned j=0; j< arraySongDirs.size(); ++j )	// for each song dir
@@ -485,6 +489,9 @@ void SongManager::LoadSongDir( RString sDir, LoadingWindow *ld, bool onlyAdditio
 
 		LOG->Trace("Loaded %i songs from \"%s\"", loaded, (sDir+sGroupDirName).c_str() );
 
+		// If we're only loading additions, already loaded groups should neither be added nor deleted
+		if (!(groupAlreadyLoaded && onlyAdditions)) {
+
 			// Don't add the group name if we didn't load any songs in this group.
 			if(!loaded) {
 				// Remove the group from the group mapping
@@ -502,6 +509,7 @@ void SongManager::LoadSongDir( RString sDir, LoadingWindow *ld, bool onlyAdditio
 
 			// Cache and load the group banner. (and background if it has one -aj)
 			IMAGECACHE->CacheImage( "Banner", GetSongGroupBannerPath(sGroupDirName) );
+		}
 
 		// Load the group sym links (if any)
 		LoadGroupSymLinks(sDir, sGroupDirName);