Shush a MSVC bool/int comparison warning.

MSVC doesn't like it when you compare the two.

Change-Id: I03c5ff2e2668ac2e536de8278e3a7c98a3dfd117
Reviewed-on: https://boringssl-review.googlesource.com/2460
Reviewed-by: Adam Langley <agl@google.com>