[secp256k1] Remove a warning in multiset test

Summary: The cast done is unsafe due to memory alignement constraints.

Test Plan: ninja check-secp256k1

Verify that the test passes and that the warning is gone.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5309