Wrong documentation for secp256k1_ec_seckey_tweak_add

[MatthewLM]

The documentation for secp256k1_ec_seckey_tweak_add states that If the tweak is invalid according to secp256k1_ec_seckey_verify, this function returns 0. However if tweak32 is zero, it will not produce an error unlike the tests in secp256k1_ec_seckey_verify so this is incorrect. It only produces an error on an overflow.

Either it should test for zero, or the documentation should be changed. A zero scalar results in the same private key. I'm not sure if allowing a zero scalar is sensible or not.