CBS_asn1_ber_to_der was a little cumbersome to use. While it, in theory,
allowed callers to consistently advance past the element, no caller
actually did so consistently. Instead they would advance if conversion
happened, and not if it was already DER. For the PKCS7_* functions, this
was even caller-exposed.

Change-Id: I658d265df899bace9ba6616cb465f19c9e6c3534
Reviewed-on: https://boringssl-review.googlesource.com/29304
Reviewed-by: Adam Langley <agl@google.com>