[boost] graph/subgraph: end iterator dereferenced

Hi, Line 189 of subgraph.hpp is: return std::make_pair((*i).second, i != m_local_vertex.end()); Consequently iterator i is dereferenced even when it is equal to m_local_vertex.end(). So here comes a patch. When *i is not valid, the patch simply returns u_global (the closest vertex_descriptor available to the code), another value may make better sense (but since the value is not supposed to be used). Regards, Guillaume