Address a timing side channel whereby it is possible to determine some

information about the length of the scalar used in ECDSA operations
from a large number (2^32) of signatures.

Thanks to Neals Fournaise, Eliane Jaulmes and Jean-Rene Reinhard for
reporting this issue.

Refer to #4576 for further details.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4623)