datapath-windows: Missed packets are not queued.

Currently, whenever there's a missed packet, the ovs driver allocates
memory and copies the packet even if there's no packet queue setup from
userspace. Then if there's no queue created, the packet is released and
dropped.

The solution was to check for the existence of the userspace queue before
trying to allocate and add a new missed packet to the queue. If there is
no userspace queue created, the original packet is dropped without creating
a new missed packet.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Nithin Raju <nithin@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/32
Tested-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>