eoslib's get_active_producers() returns wrong vector

[soft-bagel-93]

eoslib's get_active_producers() is filling vector such way using wasm-interface's same-named method:

     std::vector<name> active_prods(prod_cnt);
     get_active_producers((uint64_t*)active_prods.data(), active_prods.size());

wasm-interface's method uses memcpy, therefore it needs byte size of vector, not size().

LOW priority because this method is not used anywhere, afaik