…gain

5bf5759 bppp: Fix test for invalid sign byte again (Jonas Nick)

Pull request description:

  The first byte provided to secp256k1_bppp_parse_one_of_points is allowed to be 0, 1, 2, or 3 since it encodes the Y coordinate of two points. In a previous fix we wrongly assumed it can only be 2 or 3.

ACKs for top commit:
  real-or-random:
    utACK 5bf5759

Tree-SHA512: 4dcc83a03691b8cb0cbe39da6e34311bde518e09fb02fb8648557eba435031eaee8acef9dc762e24c7d106b935b725981dcdaadbd1a3801026ee2b7b9bc34951