Make sure we're not using an uninitialized variable in secp256k1_wnaf_const(...)
Make sure we're not using an uninitialized variable in secp256k1_wnaf_const(...)

Summary: This is a backport of secp256k1's PR533

Test Plan:
  make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4995

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