check-qjson: Simplify around compare_litqobj_to_qobj()

Make compare_litqobj_to_qobj() cope with null, and drop non-null
assertions from callers.

compare_litqobj_to_qobj() already checks the QType matches; drop the
redundant assertions from callers.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-5-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
