Also switch them to accepting a u16 length prefix. We appear not to have
any such tests right now, but RSA-2048 would involve modulus well larger
and primes just a hair larger than a u8 length prefix alows.

Change-Id: Icce8f1d976e159b945302fbba732e72913c7b724
Reviewed-on: https://boringssl-review.googlesource.com/22284
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>