dpif: Fix crash in format_odp_actions() due to NULL actions.

When flow_get fails (in this case flow does not exist) simply log
the key part of the get and erase the rest of the flow because it
is invalid.

Verified the fix by doing ovs-ofctl del-flows when traffic is running.

2014-10-18T20:12:13.785Z|00011|dpif(revalidator20)|WARN|system@ovs-system: failed to flow_get (No such file or directory) dp_hash(0),recirc_id(0),skb_priority(0),in_port(2),skb_mark(0),eth(src=00:13:72:0b:52:fa,dst=00:14:72:0b:52:fa),eth_type(0x0800),ipv4(src=10.0.0.164,dst=11.0.0.164,proto=6,tos=0,ttl=4,frag=no),tcp(src=1651,dst=6095),tcp_flags(ack), packets:0, bytes:0, used:never

Signed-off-by: Madhu Challa <challa@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>