This code reportedly upsets VC++'s static analysis. Make it clear that,
yes, we want to count backwards.

Change-Id: I5caba219a2b87750d1a9d69b46d336a98c5824c9
Reviewed-on: https://boringssl-review.googlesource.com/11624
Commit-Queue: Adam Langley <alangley@gmail.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>