openvswitch: potential NULL deref in sample()

If there is no OVS_SAMPLE_ATTR_ACTIONS set then "acts_list" is NULL and
it leads to a NULL dereference when we call nla_len(acts_list).  This
is a static checker fix, not something I have seen in testing.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>