[secp256k1] Do not use unitialized multiset in multiset benchmark

Summary:
As per title.

The compiler might possibly collapse everything based on the fact x is uninitialized, which we do not want.

Test Plan:

ninja bench-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6870