Improve detection of invalid edge descriptors

This fixes a crash when a "valid" copy of an invalid edge descriptor is
removed from the graph. Now the invalidity of the descriptor is detected
and exposed.

src/graph/graph_python_interface.cc

@@ -204,8 +204,13 @@ struct get_edge_descriptor
     {
         PythonEdge<Graph>& pe = python::extract<PythonEdge<Graph>&>(e);
         pe.CheckValid();
-        edge = pe.GetDescriptor();
         pe.SetValid(false);
+        typename graph_traits<Graph>::out_edge_iterator e_begin, e_end;
+        tie(e_begin, e_end) = out_edges(source(pe.GetDescriptor(),g),g);
+        e_begin = std::find(e_begin, e_end, pe.GetDescriptor());
+        if (e_begin == e_end)
+            return; // invalid edge descriptor
+        edge = pe.GetDescriptor();
         found = true;
     }
 };

src/graph/graph_python_interface.hh

@@ -249,11 +249,9 @@ public:
         if (_g().ptr() == Py_None)
             return false;
         GraphInterface& gi = python::extract<GraphInterface&>(_g());
-        return _valid &&
-            (gi._vertex_index[target(_e, gi._mg)] < num_vertices(gi._mg)) &&
-            (gi._vertex_index[source(_e, gi._mg)] < num_vertices(gi._mg)) &&
-            (target(_e, gi._mg) != graph_traits<Graph>::null_vertex()) &&
-            (source(_e, gi._mg) != graph_traits<Graph>::null_vertex());
+        GraphInterface::edge_t e(_e);
+        return (_valid && PythonVertex(_g, source(e, gi._mg)).IsValid() &&
+                PythonVertex(_g, target(e, gi._mg)).IsValid());
     }
 
     void SetValid(bool valid)
@@ -263,7 +261,7 @@ public:
 
     void CheckValid() const
     {
-        if (!_valid)
+        if (!IsValid())
             throw ValueException("invalid edge descriptor");
     }