if we get empty brushes in CSG subtract, don't complain, just don't insert the new brush (as this can happen intentionally when a brush is entirely covered)

radiant/csg.cpp

@@ -397,13 +397,17 @@ public:
           for(brush_vector_t::const_iterator i = out.begin(); i != out.end(); ++i)
           {
             ++m_after;
-            NodeSmartReference node((new BrushNode())->node());
             (*i)->removeEmptyFaces();
-            ASSERT_MESSAGE(!(*i)->empty(), "brush left with no faces after subtract");
+            if(!(*i)->empty())
+            {
+              NodeSmartReference node((new BrushNode())->node());
               Node_getBrush(node)->copy(*(*i));
               delete (*i);
               Node_getTraversable(path.parent())->insert(node);
             }
+            else
+              delete (*i);
+          }
           Path_deleteTop(path);
         }
       }