It's subtle, since it is actually only touched by hashfp (though
we assert it's non-NULL), but give explicit advice in the default
case.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This is a backport of libsecp256k1 [[ https://github.com/bitcoin-core/secp256k1/pull/714 | PR714 ]].