Timing vulnerability in DSA signature generation (CVE-2018-0734).

Avoid a timing attack that leaks information via a side channel that
triggers when a BN is resized.  Increasing the size of the BNs
prior to doing anything with them suppresses the attack.

Thanks due to Samuel Weiser for finding and locating this.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7486)

(cherry picked from commit a9cfb8c2)