try_len
valid[0] = try_len(space_left, orig_len, base_hdr_len,
valid[1] = try_len(space_left, orig_len, base_hdr_len,
valid[2] = try_len(space_left, orig_len, base_hdr_len,
valid[3] = try_len(space_left, orig_len, base_hdr_len,