ofproto-dpif: Don't output to nonexistent ports.

ofproto-dpif: Don't output to nonexistent ports.

In older versions of Open vSwitch, one could reasonably predict
that the datapath port number would be the same as the OpenFlow
port number even for ports which may not exist yet.  With the
single datapath model, it's no possible longer to make this
prediction.  Therefore, instead of attempting to, this code simply
drops packets which output to a nonexistent port.

Signed-off-by: Ethan Jackson <ethan@nicira.com>