bn_get_bits5 always reads two bytes, even when it doesn't need to. For some
sizes of |p|, this can result in reading just past the edge of the array.
Unroll the first iteration of the loop and avoid reading out of bounds.

Replace bn_get_bits5 altogether in C as it's not doing anything interesting.

Change-Id: Ibcc8cea7d9c644a2639445396455da47fe869a5c
Reviewed-on: https://boringssl-review.googlesource.com/1393
Reviewed-by: Adam Langley <agl@google.com>