Merge #20257: Update secp256k1 subtree to latest master

6c0259fc2f8bd34ba83ad10a6a11d6d99e8d1fc7 Squashed 'src/secp256k1/' changes from c6b6b8f1bb..3967d96bf1 (Pieter Wuille)

Pull request description:

  Nothing important changed, but this silences this (erroneous) warning in certain GCC 9 versions:

  ```
  In file included from src/secp256k1.c:16:
  src/ecmult_impl.h: In function ‘secp256k1_ecmult’:
  src/ecmult_impl.h:496:48: warning: array subscript [1, 268435456] is outside array bounds of ‘struct secp256k1_strauss_point_state[1]’ [-Warray-bounds]
    496 |             secp256k1_gej tmp = a[state->ps[np].input_pos];
        |                                   ~~~~~~~~~~~~~^~~~~~~~~~
  src/ecmult_impl.h:565:42: note: while referencing ‘ps’
    565 |     struct secp256k1_strauss_point_state ps[1];
        |                                          ^~
  src/ecmult_impl.h:502:139: warning: array subscript [1, 268435456] is outside array bounds of ‘struct secp256k1_strauss_point_state[1]’ [-Warray-bounds]
    502 |             secp256k1_fe_mul(state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), &(a[state->ps[np].input_pos].z));
        |                                                                                                                              ~~~~~~~~~~~~~^~~~~~~~~~
  src/ecmult_impl.h:565:42: note: while referencing ‘ps’
    565 |     struct secp256k1_strauss_point_state ps[1];
        |                                          ^~
  ```

  (see https://github.com/bitcoin-core/secp256k1/issues/834)

ACKs for top commit:
  fanquake:
    ACK 5803f5f5f6030e69b46a46f0511b8173bf89de0d  - performed the update myself and got the same change: [check_20257_subtree](https://github.com/fanquake/bitcoin/tree/check_20257_subtree).
  hebasto:
    ACK 5803f5f5f6030e69b46a46f0511b8173bf89de0d, tested on Linux Mint 20 (x86_64) with `gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0` -- no warnings are emitted.

Tree-SHA512: 386281d23aee93a3b1d1a09fec8319c3a477e46967430c935677eed54abddc62d5a7710f9eeab1ec476ace05adcb194b5b377712e44a6bb95a74ffa35faf77f3

0 files changed, 0 insertions, 0 deletions