Check for NULL conf in NCONF_get_number

The problematic case falls back to a NULL conf which returns the result
of getenv(2).  If this returns NULL, everything was good.  If this returns
a string an attempt to convert it to a number is made using the function
pointers from conf.

This fix uses the strtol(3) function instead, we don't have the
configuration settings and this behaves as the default would.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6632)