If ssl_do_write takes more than one iteration, ssl3_output_cert_chain
would be called an extra time. This is very unlikely in practice because
of the buffer BIO.

Change-Id: Ic1ae9752a8837bb404429fc60306c659208c6185
Reviewed-on: https://boringssl-review.googlesource.com/5340
Reviewed-by: Adam Langley <agl@google.com>