Revert "ecdsa_impl: replace scalar if-checks with VERIFY_CHECKs in ecdsa_sig_sign"
Revert "ecdsa_impl: replace scalar if-checks with VERIFY_CHECKs in ecdsa_sig_sign"

This reverts commit 25e3cfbf9b52d2f5afa543f967a73aa8850d2038. The reverted
commit was probably based on the assumption that this is about the touched
checks cover the secret nonce k instead of r, which is the x-coord of the public
nonce. A signature with a zero r is invalid by the spec, so we should return 0
to make the caller retry with a different nonce. Overflow is not an issue.

Fixes #720.

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