Switch ssl3_send_channel_id to BN_bn2bin_padded.

Check the return value while we're here. This avoids some arithmetic and
appease scan-build's dead assignment flagger.

Change-Id: If3615076e091eb44b9e3e9d50cd64f80e645337e
Reviewed-on: https://boringssl-review.googlesource.com/2204
Reviewed-by: Adam Langley <agl@google.com>