message: Use size_t shifts when computing a size_t
  Summary: This is a backport of secp256k1's PR607
  Test Plan:
  make check
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -960,7 +960,7 @@ static size_t secp256k1_pippenger_scratch_size(size_t n_points, int bucket_windo
  size_t entries = n_points + 1;
  #endif
  size_t entry_size = sizeof(secp256k1_ge) + sizeof(secp256k1_scalar) + sizeof(struct secp256k1_pippenger_point_state) + (WNAF_SIZE(bucket_window+1)+1)*sizeof(int);
  - return ((1<<bucket_window) * sizeof(secp256k1_gej) + sizeof(struct secp256k1_pippenger_state) + entries * entry_size);
  + return (sizeof(secp256k1_gej) << bucket_window) + sizeof(struct secp256k1_pippenger_state) + entries * entry_size;
  }
  static int secp256k1_ecmult_pippenger_batch(const secp256k1_ecmult_context *ctx, secp256k1_scratch *scratch, secp256k1_gej *r, const secp256k1_scalar *inp_g_sc, secp256k1_ecmult_multi_callback cb, void *cbdata, size_t n_points, size_t cb_offset) {
  @@ -996,7 +996,7 @@ static int secp256k1_ecmult_pippenger_batch(const secp256k1_ecmult_context *ctx,
  state_space = (struct secp256k1_pippenger_state *) secp256k1_scratch_alloc(scratch, sizeof(*state_space));
  state_space->ps = (struct secp256k1_pippenger_point_state *) secp256k1_scratch_alloc(scratch, entries * sizeof(*state_space->ps));
  state_space->wnaf_na = (int *) secp256k1_scratch_alloc(scratch, entries*(WNAF_SIZE(bucket_window+1)) * sizeof(int));
  - buckets = (secp256k1_gej *) secp256k1_scratch_alloc(scratch, (1<<bucket_window) * sizeof(*buckets));
  + buckets = (secp256k1_gej *) secp256k1_scratch_alloc(scratch, sizeof(*buckets) << bucket_window);
  if (inp_g_sc != NULL) {
  scalars[0] = *inp_g_sc;
  @@ -1063,7 +1063,7 @@ static size_t secp256k1_pippenger_max_points(secp256k1_scratch *scratch) {
  #ifdef USE_ENDOMORPHISM
  entry_size = 2*entry_size;
  #endif
  - space_overhead = ((1<<bucket_window) * sizeof(secp256k1_gej) + entry_size + sizeof(struct secp256k1_pippenger_state));
  + space_overhead = (sizeof(secp256k1_gej) << bucket_window) + entry_size + sizeof(struct secp256k1_pippenger_state);
  if (space_overhead > max_alloc) {
  break;
  }
  