Change some "int" variables to "size_t" in ssl3_handshake_mac().

BUG=https://crbug.com/535039

Change-Id: I94d35ae5acee510b1e6874c37f35eddda9906e71
Reviewed-on: https://boringssl-review.googlesource.com/5974
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>