Fix T86298: crash when loading "corrupted" geometry node tree

The file was not really corrupted (as in, Blender did everything
correctly while saving). I only did not consider the case when
a .blend file is resaved in an older version before.

source/blender/blenkernel/intern/node.cc

@@ -778,6 +778,13 @@ static void lib_link_node_socket(BlendLibReader *reader, Library *lib, bNodeSock
 {
   IDP_BlendReadLib(reader, sock->prop);
 
+  /* This can happen for all socket types when a file is saved in an older version of Blender than
+   * it was originally created in (T86298). Some socket types still require a default value. The
+   * default value of those sockets will be created in `ntreeSetTypes`. */
+  if (sock->default_value == nullptr) {
+    return;
+  }
+
   switch ((eNodeSocketDatatype)sock->type) {
     case SOCK_OBJECT: {
       bNodeSocketValueObject *default_value = (bNodeSocketValueObject *)sock->default_value;