message: Moved a dereference so the null check will be before the dereferencing
  Summary: This is a backport of libsecp256k1 PR644
  Test Plan:
  ninja check-secp256k1
  Reviewers: #bitcoin_abc, nakihito
diff: @@ -395,7 +395,7 @@ static void secp256k1_ecmult_context_clear(secp256k1_ecmult_context *ctx) {
  * than the number of bits in the (absolute value) of the input.
  */
  static int secp256k1_ecmult_wnaf(int *wnaf, int len, const secp256k1_scalar *a, int w) {
  - secp256k1_scalar s = *a;
  + secp256k1_scalar s;
  int last_set_bit = -1;
  int bit = 0;
  int sign = 1;
  @@ -408,6 +408,7 @@ static int secp256k1_ecmult_wnaf(int *wnaf, int len, const secp256k1_scalar *a,
  memset(wnaf, 0, len * sizeof(wnaf[0]));
  + s = *a;
  if (secp256k1_scalar_get_bits(&s, 255, 1)) {
  secp256k1_scalar_negate(&s, &s);
  sign = -1;
  