[secp256k1] Enable endomorphism by default

Summary:
The patent on this optimisation expired, so it can be enabled by default now.

Also fix a warning about possibly uninitialized variable.

Test Plan:

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