Ensure named_graph base class is initialized before accessing it

[AnthonyVH]

This PR fixes the issue that a boost::adjacency_list does not properly initialize it's graph::maybe_named_graph base before calling functions on it.

More specifically, the following crashes:

using namespace boost;

// Assume something::Vertex has proper specializations for `boost::internal_vertex_name`
// and `boost::internal_vertex_name`.
using graph_t = adjacency_list<hash_setS, vecS, bidirectionalS, something::Vertex>;

graph_t graphA;
// Initialize `graphA` here.

graph_t graphB{graphA}; // Boom!

The reason this happens is because:

• boost::adjacency_list inherits from detail::adj_list_gen<...> and graph::maybe_named_graph<...>
• boost::adjacency_list typedefs detail::adj_list_gen<...> as Base
• The copy constructor calls Base(x) as its very first "action", where x is graphA
• This in turn ends up calling vec_adj_list_impl<...>(graphA), which calls copy_impl(graphA)
• This in turn calls boost::add_vertex on adjacency_list & (which vec_adj_list_impl can cast itself to due to CRTP)
• This in turn calls vec_adj_list_impl::added_vertex() which is "injected" into adjacency_list via the graph::maybe_named_graph inheritance.

When this last call (added_vertex()) happens on graphs that are not named graphs, nothing happens (i.e. added_vertex() is a no-op). However, it does do a bunch of stuff if the graph is a named graph (and thus inherits from graph::named_graph. In that case there's some uninitialized size member which hasn't been initialized yet, which gets used at some point and then the whole thing blows up.

The fix is simple: just inherit from graph::maybe_named_graph first, such that the call to Base(x) happens after the graph::maybe_named_graph base has already been default initialized.

Note that there's no move constructor, i.e. the copy constructor will always get called even if an rvalue graph_t is passed to the constructor. So the above also happens for e.g. graph_t graphB{getGraph()}.

[jeremy-murphy]

Thanks for making an incremental improvement to the quality of this library!