[5.0] Return error for num == 0 for bls_g1_weighted_sum, bls_g2_weighted_sum and bls_pairing

[heifner]

Return error for num == 0 for host functions bls_g1_weighted_sum, bls_g2_weighted_sum, and bls_pairing.

Resolves #1953

[linh2931]

n == 0 tests need to be updates.

[greg7mdp]

Not related to your change, and probably not something we want to do in 5.0, but maybe in the future we can make the code a little cleaner by converting the eos span<char> -> std::span<uint8_t> only once, as in:

   int32_t interface::bls_g1_weighted_sum(span<const char> _points, span<const char> _scalars, const uint32_t n, span<char> result) const {
      if(n == 0 || _points.size() != n*96 ||  _scalars.size() != n*32 ||  result.size() != 96)
         return return_code::failure;

      std::span<const uint8_t> points((const uint8_t*)_points.data(), n*96);
      std::span<const uint8_t> scalars((const uint8_t*)_scalars.data(), n*32);

      // Use much efficient scale for the special case of n == 1.
      if (1 == n) {
         std::optional<bls12_381::g1> a = bls12_381::g1::fromAffineBytesLE(points.first<96>(), true, false);
         if(!a)
            return return_code::failure;
         std::array<uint64_t, 4> b = bls12_381::scalar::fromBytesLE<4>(scalars.first<32>());
         bls12_381::g1 c = a->scale(b);
         c.toAffineBytesLE(std::span<uint8_t, 96>((uint8_t*)result.data(), 96), false);
         return return_code::success;
      }

      std::vector<bls12_381::g1> pv;
      std::vector<std::array<uint64_t, 4>> sv;
      pv.reserve(n);
      sv.reserve(n);
      for(uint32_t i = 0; i < n; i++)
      {
         std::optional<bls12_381::g1> p = bls12_381::g1::fromAffineBytesLE(points.subspan(i*96).first<96>(), true, false);
         if(!p.has_value())
            return return_code::failure;
         std::array<uint64_t, 4> s = bls12_381::scalar::fromBytesLE<4>(scalars.subspan(i*32).first<32>());
         pv.push_back(p.value());
         sv.push_back(s);
         if(i%10 == 0)
            context.trx_context.checktime();
      }
      bls12_381::g1 r = bls12_381::g1::weightedSum(pv, sv, [this](){ context.trx_context.checktime();}); // accessing value is safe
      r.toAffineBytesLE(std::span<uint8_t, 96>((uint8_t*)result.data(), 96), false);
      return return_code::success;
   }

[heifner]

I don't see a code style change like this appropriate at this point.