ofproto-dpif-xlate: Fix null pointer dereference

actions (in xlate_actions__) would be NULL when xlate_actions()
is called from packet_out()->ofproto_dpif_execute_actions().
This causes a NULL pointer to be dereferenced when
ctx.xbridge->netflow is set.

Signed-off-by: Anoob Soman <anoob.soman@citrix.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>