if tag type for column is invalid, display empty column instead of crashing

src/display.cpp

@@ -84,6 +84,7 @@ std::string Display::Columns()
 				tag = "Comment";
 				break;
 			default:
+				tag.clear();
 				break;
 		}
 		if (it->right_alignment)
@@ -182,7 +183,7 @@ void Display::SongsInColumns(const MPD::Song &s, void *, Menu<MPD::Song> *menu)
 		if (it->color != clDefault)
 			*menu << it->color;
 		whline(menu->Raw(), 32, menu->GetWidth()-where);
-		std::string tag = (s.*get)();
+		std::string tag = get ? (s.*get)() : "";
 		if (it->right_alignment)
 		{
 			if (!tag.empty() || it->display_empty_tag)

src/settings.cpp

@@ -919,9 +919,15 @@ void ReadConfiguration(ncmpcpp_config &conf)
 		Column col;
 		col.color = IntoColor(GetLineValue(conf.song_list_columns_format, '[', ']', 1));
 		std::string tag_type = GetLineValue(conf.song_list_columns_format, '{', '}', 1);
-		col.type = tag_type.at(0);
+		if (tag_type.length() > 0) // at least tag type was specified
+			col.type = tag_type[0];
+		else
+		{
+			col.type = 0;
+			col.display_empty_tag = 0;
+		}
 		col.fixed = *width.rbegin() == 'f';
-		for (std::string::const_iterator it = ++tag_type.begin(); it != tag_type.end(); ++it)
+		for (std::string::const_iterator it = tag_type.begin()+(tag_type.length() > 0); it != tag_type.end(); ++it)
 		{
 			switch (*it)
 			{