Fix Windows build.

It (incorrectly) thinks some variables are uninitialized. It also gets confused
about some const parameters.

Change-Id: Ie2b3a5336692e7293cf03d6a4cd5345d30b628b3
Reviewed-on: https://boringssl-review.googlesource.com/5330
Reviewed-by: Adam Langley <agl@google.com>