Fix the return type of felem_is_zero_int which should be int.

Change argument type of xxxelem_is_zero_int to const void*
to avoid the need of type casts.

Fixes #4413

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4450)

(cherry picked from commit c55b786a)