Fix uninit values passed into cmov

Summary:

• Fixed UB(arithmetics on uninit values) in cmovs
• Add valgrind uninit check to cmovs output

This is a backport of libsecp256k1 PR754

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7589