NCONF_get_number refix.

Fix the NULL check lack in a different way that is more compatible with
non-NULL branch.  Refer #6632

Also mark and pop the error stack instead of clearing all errors when something
goes awry in CONF_get_number.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6643)