Fix: 'openssl ca' command crashes when used with 'rand_serial' option

Commit ffb46830

 introduced the 'rand_serial' option. When it is used,
the 'serialfile' does not get initialized, i.e. it remains a NULL pointer.
This causes a crash when the NULL pointer is passed to the rotate_serial()
call.

This commit fixes the crash and unifies the pointer checking before
calling the rotate_serial() and save_serial() commands.

Fixes #7412

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7417)