bug in Detour.getTileByRef

[nikita-one]

Hi. There is a bug in Detour.getTileByRef. The first check against 0 should be flipped:

       public dtMeshTile getTileByRef(dtTileRef tileRef)
        {
            if (tileRef == 0) <--- here 
                return null;

[nikita-one]

Otherwise a good port! I had to fiddle a bit with Detour to remove all allocations, but it still saved me a lot of time.