Fix #124463: crash when trying link a node to itself with swapping

The swapping code incorrectly expected the link to have a start and end.
However, this was not necessarily the case when attempting to create
a link from a node to itself. This case has special handling in
`node_link_find_socket`.

source/blender/editors/space_node/node_intern.hh

@@ -46,6 +46,7 @@ struct NestedTreePreviews;
 struct bNodeLinkDrag {
   /** Links dragged by the operator. */
   Vector<bNodeLink> links;
+  /** Which side if the links is fixed. */
   eNodeSocketInOut in_out;
 
   /** Draw handler for the tooltip icon when dragging a link in empty space. */

source/blender/editors/space_node/node_relationships.cc

@@ -1087,6 +1087,9 @@ static void displace_links(bNodeTree *ntree, const bNode *node, bNodeLink *inser
 static void node_displace_existing_links(bNodeLinkDrag &nldrag, bNodeTree &ntree)
 {
   bNodeLink &link = nldrag.links.first();
+  if (!link.fromsock || !link.tosock) {
+    return;
+  }
   if (nldrag.start_socket->is_input()) {
     displace_links(&ntree, link.fromnode, &link);
   }