Fix SSL_CTX_get_{min,max}_proto_version integer conversion warning

When using the SSL_CTX_get_min_min_version macro while compiling with
-Wall, my compiler rightfully complains about this construction:

warning: passing argument 3 of ‘SSL_CTX_ctrl’ makes integer from
pointer without a cast [-Wint-conversion]

These macro's should use 0, instead of NULL, for the third argument,
like most other SSL_CTX_ctrl 'get' wrappers do.

CLA: trivial
Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/5099)