Explicitly check for empty certificate list.

The NULL checks later on notice, but failing with
SSL_R_UNABLE_TO_FIND_PUBLIC_KEY_PARAMETERS on accident is confusing.
Require that the message be non-empty.

Change-Id: Iddfac6a3ae6e6dc66c3de41d3bb26e133c0c6e1d
Reviewed-on: https://boringssl-review.googlesource.com/5046
Reviewed-by: Adam Langley <agl@google.com>
4 files changed