fix maybe-unused warning in grid_tools_dofhandler

[tjhei]

./include/deal.II/grid/reference_cell.h:2799:22: warning: ‘face1_vertices.std::array<unsigned[763/12022] M_elems[0]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
./source/grid/grid_tools_dof_handlers.cc:2438:7: note: ‘face1_vertices.std::array<unsigned int, 1>::_M_elems[0]’ was declared here

[bangerth]

I understand why this patch fixes the warning, but I don't understand why this is the right thing to do. In the code below, down to line 2475, face1_vertices and face2_vertices seem to be written into in always the same places. So it's not clear to me why initializing only one of these but not the other is right.

But really what's unclear to me is why initialization with zero is right. Do we expect every element of the array to be written into? If so, perhaps initialization with an invalid value would be the right choice?

[tjhei]

I assume the diagnostics just pick up the loop iterating over face1_vertices in the other function, but I don't know for sure.
I don't understand the code fully, but I now fill both with invalid data and check the content at the end. We'll see what happens.

[bangerth]

Yes, this is better. I'm confounded by you choosing obj.end() == std::find(...) over std::find(...) == std::end() :-)