Timing vulnerability in ECDSA signature generation (CVE-2018-0735)

Preallocate an extra limb for some of the big numbers to avoid a reallocation
that can potentially provide a side channel.

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