Fix signature of min/max proto getter

The getters for min and max proto version wrongly passed NULL instead of
0 as third argument to SSL_ctrl() and SSL_CTX_ctrl(). The third argument
is not used, but the error results in a compiler warning:

warning: passing argument 3 of ‘SSL_CTX_ctrl’ makes integer from pointer without a cast [-Wint-conversion]
int v = SSL_CTX_get_max_proto_version(self->ctx);

See https://github.com/openssl/openssl/pull/4364



Signed-off-by: Christian Heimes <christian@python.org>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/5128)