Fix Build: mcrouter under Clang with NDEBUG fails after D2505429
Fix Build: mcrouter under Clang with NDEBUG fails after D2505429

Summary: [MCRouter] Fix Build: `mcrouter` under Clang with `NDEBUG` fails after D2505429.

The problem is that there is a local variable that is only written if `NDEBUG` and never read, and Clang warns.

Test Plan: CI.

Reviewed By: spalamarchuk

Differential Revision: D2516588

fb-gh-sync-id: 9aa2dc168f14b21338bd02b94625bf9188d31e14

yqzhang/mcrouter,facebook/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,yqzhang/mcrouter,facebook/mcrouter,nvaller/mcrouter,nvaller/mcrouter,reddit/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,facebook/mcrouter,nvaller/mcrouter,facebook/mcrouter,synecdoche/mcrouter,reddit/mcrouter,reddit/mcrouter,reddit/mcrouter,synecdoche/mcrouter,nvaller/mcrouter