flow_dissector: do not break if ports are not needed in flowlabel

This restored previous behaviour. If caller does not want ports to be
filled, we should not break.

Fixes: 06635a35

 ("flow_dissect: use programable dissector in skb_flow_dissect and friends")
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>