Fix TLS 1.2 and no sigalgs.

For TLS 1.2 if we have no signature algorithms extension then lookup
using the complete table instead of (empty) shared signature algorithms
list so we pick up defaults.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2301)