message: Make sure we're not using an uninitialized variable in secp256k1_wnaf_const(...)
  Summary: This is a backport of secp256k1's PR533
  Test Plan:
  make check
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -61,6 +61,10 @@ static int secp256k1_wnaf_const(int *wnaf, const secp256k1_scalar *scalar, int w
  int bit;
  secp256k1_scalar s;
  int not_neg_one;
  +
  + VERIFY_CHECK(w > 0);
  + VERIFY_CHECK(size > 0);
  +
  /* Note that we cannot handle even numbers by negating them to be odd, as is
  * done in other implementations, since if our scalars were specified to have
  * width < 256 for performance reasons, their negations would have width 256
  @@ -94,7 +98,7 @@ static int secp256k1_wnaf_const(int *wnaf, const secp256k1_scalar *scalar, int w
  /* 4 */
  u_last = secp256k1_scalar_shr_int(&s, w);
  - while (word * w < size) {
  + do {
  int sign;
  int even;
  @@ -110,7 +114,7 @@ static int secp256k1_wnaf_const(int *wnaf, const secp256k1_scalar *scalar, int w
  wnaf[word++] = u_last * global_sign;
  u_last = u;
  - }
  + } while (word * w < size);
  wnaf[word] = u * global_sign;
  VERIFY_CHECK(secp256k1_scalar_is_zero(&s));
  