Fix for missing integer 'type' of the virtual extension socket in group nodes. This could lead to crash in node groups when trying to undefined stack values of the extension socket. type just needs to be set to SOCK_CUSTOM==-1, so the BI execution knows not to look for a stack entry.

source/blender/nodes/intern/node_socket.c

@@ -420,6 +420,9 @@ static bNodeSocketType *make_socket_type_virtual(void)
 	/* associate the RNA type with the socket type */
 	RNA_struct_blender_type_set(srna, stype);
 	
+	/* extra type info for standard socket types */
+	stype->type = SOCK_CUSTOM;
+	
 	ED_init_node_socket_type_virtual(stype);
 	
 	return stype;