fix bug in fread() failure check
fix bug in fread() failure check

the two middle arguments to fread() are easily confused, and cause the
checking of return value to fail incorrectly (and possibly succeed
incorrectly.)

ElementsProject/secp256k1-zkp,afk11/secp256k1,bitcoin/secp256k1,afk11/secp256k1,libbitcoin/secp256k1,evoskuil/secp256k1,pmienk/secp256k1,peterdettman/secp256k1,afk11/secp256k1,pmienk/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,libbitcoin/secp256k1,bitcoin-core/secp256k1,peterdettman/secp256k1,bitcoin-core/secp256k1,libbitcoin/secp256k1,pmienk/secp256k1,evoskuil/secp256k1,pmienk/secp256k1,ElementsProject/secp256k1-zkp,afk11/secp256k1,pmienk/secp256k1,evoskuil/secp256k1,evoskuil/secp256k1,libbitcoin/secp256k1,libbitcoin/secp256k1,evoskuil/secp256k1,afk11/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1