[secp256k1] Enable endomorphism by default
The patent on this optimisation expired, so it can be enabled by default now.
Also fix a warning about possibly uninitialized variable.
ninja all check-all
Ran the extended test suite under ubsan and did run IBD + kept node runnign under ubsan for testnet and mainnet over night.
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7971