Handle unused boundary nodes correctly

For "unused" boundary nodes the special value `-1` is used.  Mapping
this through `isBoundaryNode[-1]` would result in an out-of-bounds
memory access and invalid values in the `vertices_c_style` array.

Closes: #55

dune/grid/uggrid/uggridfactory.cc

@@ -322,8 +322,12 @@ createGrid()
     // Copy the vertices into a C-style array
     // We copy four vertices here even if the segment is a triangle -- it doesn't matter
     int vertices_c_style[dimworld*2-2];
-    for (int j=0; j<dimworld*2-2; j++)
-      vertices_c_style[j] = isBoundaryNode[boundarySegmentVertices_[i][j]];
+    for (int j=0; j<dimworld*2-2; j++) {
+      // For parameterized boundary segments, -1 is used as a sentinel value
+      // for unused vertices and must be preserved.
+      const auto idx = boundarySegmentVertices_[i][j];
+      vertices_c_style[j] = idx == -1 ? -1 : isBoundaryNode[idx];
+    }
 
     if (grid_->boundarySegments_[i]) {