grid_vertex_converter: Fix handling of empty polygon

include/mapnik/grid_vertex_converter.hpp

@@ -167,6 +167,8 @@ private:
     }
 
     double get_hit_bitmap_scale(box2d<T> const& envelope) const
+    {
+        if (envelope.valid())
         {
             T size = envelope.width() * envelope.height();
             // Polygon with huge area can lead to excessive memory allocation.
@@ -177,6 +179,7 @@ private:
             {
                 return std::sqrt(max_size / size);
             }
+        }
         return 1;
     }
 
@@ -212,6 +215,8 @@ private:
                                         double scale_factor) const
     {
         mapnik::geometry::point<T> interior;
+        if (envelope.valid())
+        {
             if (!mapnik::geometry::interior(poly, scale_factor, interior))
             {
                 auto center = envelope.center();
@@ -220,7 +225,7 @@ private:
             }
 
             vt_.forward(&interior.x, &interior.y);
-
+        }
         return interior;
     }