message: Use ptrdiff_t type to more precisely indicate usage and avoid compiler warnings
diff: @@ -121,7 +121,7 @@ static int ec_privkey_import_der(const secp256k1_context *ctx,
  {
  return 0;
  }
  - size_t lenb = *privkey & ~0x80u;
  + ptrdiff_t lenb = *privkey & ~0x80u;
  privkey++;
  if (lenb < 1 || lenb > 2)
  {
  @@ -132,7 +132,7 @@ static int ec_privkey_import_der(const secp256k1_context *ctx,
  return 0;
  }
  /* sequence length */
  - size_t len = privkey[lenb - 1] | (lenb > 1 ? privkey[lenb - 2] << 8 : 0u);
  + ptrdiff_t len = privkey[lenb - 1] | (lenb > 1 ? privkey[lenb - 2] << 8 : 0u);
  privkey += lenb;
  if (end - privkey < len)
  {
  @@ -149,7 +149,7 @@ static int ec_privkey_import_der(const secp256k1_context *ctx,
  {
  return 0;
  }
  - size_t oslen = privkey[1];
  + ptrdiff_t oslen = privkey[1];
  privkey += 2;
  if (oslen > 32 || end - privkey < oslen)
  {
  