Treat 0 the same as -1 and NULL for numeric columns such as "disc".

Update issue #411
Status: Fixed
Ah ha!  I knew it was the disc tag - one of them had a value of 0 set, and the other had no value at all which was taken by clementine to mean "-1", which was lower than 0 so screwed up the sorting.  Try now and it should be fixed (you'll have to get Clementine to rescan that directory - probably remove it from the library settings and add it again).

src/core/song.cpp

@@ -536,7 +536,7 @@ void Song::MergeFromSimpleMetaBundle(const Engine::SimpleMetaBundle &bundle) {
 }
 
 void Song::BindToQuery(QSqlQuery *query) const {
-  #define intval(x) (x == -1 ? QVariant() : x)
+  #define intval(x) (x <= 0 ? QVariant() : x)
 
   // Remember to bind these in the same order as kBindSpec