Fix: failed to open config file if not specified when using CA commands

Issue was introduced in
https://github.com/openssl/openssl/commit/a0a82324f965bbcc4faed4e1ee3fcaf81ea52166



This patch fixes an issue which causes the 'openssl ca' commands to
fail if '-config' is not specified even if it says so otherwise.
Problem is that the default config is not loaded and the conf variable
is NULL which causes an exception.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>