Fix for InteriorInstance's that only have NULL surfaces

A small bug crept into TGE 1.5.1 and TGE 1.5.2 where if you try to add an InteriorInstance that only has NULL surfaces or an InteriorInstance that only uses Static Meshes and no brushes to a scene that already has an InteriorInstance in it, it would cause a hard crash.

This was caused because the lack of any visible surfaces would cause Interior::setupActivePolyList() to return early and some of the global variables that Interior uses for rendering would never get reset back to zero.

Simply add these lines to the top of Interior::setupActivePolyList() in engine\interior\interior.cc so that it looks like this:

void Interior::setupActivePolyList(ZoneVisDeterminer& zoneDeterminer,
                                   SceneState*        state,
                                   const Point3F&     rPoint,
                                   const Point3F&     osCamVector,
                                   const Point3F&     osZVec,
                                   const F32          worldZ,
                                   const Point3F&     scale)
{
   // These global variables need to be initialized in case we return early
   sgActivePolyListSize  = 0;
   sgEnvironPolyListSize = 0;
   sgFogPolyListSize     = 0;