datapath: Zero out key when looking up null ports.

datapath: Zero out key when looking up null ports.

When we are searching for a tunnel port to receive traffic on,
everything should be zeroed out by the time that we get to null
ports since they are wildcarded.  However, if certain other ports
also exist (primarily multicast ports with keys) then this might
not be the case and the key can be set.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>