add missing swap to CScriptCheck

Summary:
My bad, I missed adding this. It doesn't really matter since
the results of CScriptCheck (neither metrics nor error) are
never used in the parallel setting. In future I'd really like
to make error/metrics not be members of this class anyway,
instead to be optional outparams of the operator().

Test Plan: ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5225