[clang-tidy] Fix unused-variable warning after r361647.

Summary:
A range-for was added in r361647 where the range variable was only used in an
assertion.  As a result, it warned for Release builds. This revision
restructures the assertion to avoid the problem.

Patch by Yitzhak Mandelbaum.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: xazax.hun, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D62412

llvm-svn: 361749