Document failure return for ECDSA_SIG_new

ECDSA_SIG_new() returns NULL on error.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6398)