Use BN_bn2bin_padded rather than doing math to figure out leading zeros.

Saves doing it ad-hoc all the time.

Change-Id: Ic1a1180f56eec37c19799649bb8f18237bd617f8
Reviewed-on: https://boringssl-review.googlesource.com/2241
Reviewed-by: Adam Langley <agl@google.com>