Add a maximum span length check to XTP. Fixes a crash discovered by Ben Schmidt. Just use proto_tree_add_item() instead of allocating a big-ol-honkin'-block of guint64s and adding their values to the tree via proto_tree_add_uint64(). Dissection *should* still work correctly but I can't find any XTP captures for testing.
trunk/epan/dissectors/packet-xtp.c
