Added segfault protection for edge in Relationship::disconnector.

Author: Tom Manner

src/relationship.h

@@ -84,7 +84,7 @@ class Relationship {
     void operator()(DiGraph* digraph, tail_t* tail, head_t* head) {
       if (tail and head) {
         Edge<tail_t, head_t>* edge = tail->Relationship<tail_t, head_t>::getOutgoingEdge();
-        if (edge->getHead() == head) {
+        if (edge and edge->getHead() == head) {
           tail->Relationship<tail_t, head_t>::disconnect(edge);
           tail->removeEdge(edge);
           head->Relationship<tail_t, head_t>::disconnect(edge);