Make 'numVertices' work for uninitialised patches by returning zero

dune/fufem/boundarypatch.hh

@@ -616,6 +616,8 @@ public:
 
     /** \brief Returns the number of vertices in the surface */
     int numVertices() const {
+        if (gridView_ == nullptr)
+            return 0;
         if (!this->verticesAreUpToDate_)
             this->makeVertices();