qapi: Make QObject input visitor set *list reliably

qapi: Make QObject input visitor set *list reliably

qobject_input_start_struct() sets *list, except when it fails because
qobject_input_get_object() fails, i.e. the input object doesn't exist.

All the other input visitor start_struct(), start_list(),
start_alternate() always set *obj / *list.

Change qobject_input_start_struct() to match.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1488544368-30622-14-git-send-email-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>