Calling subgraph::global_to_local on a root graph

[nobody]

When g is a root subgraph,
then g.global_to_local(g.local_to_global(v))
will cause an error.
In Fact, the mapping m_local_vertex is not
maintained for root subgraphs,
so global_to_local and find_vertex would
not work correctly.

Thanks for fixing it.