Fix MSVC build.

The difference of two pointers is signed, even though it's always
non-negative here, so MSVC is complaining about signedness mismatch.

Change-Id: I5a042d06ed348540706b93310af3f60f3ab5f303
Reviewed-on: https://boringssl-review.googlesource.com/5766
Reviewed-by: Adam Langley <agl@google.com>
1 file changed