datapath-windows: Fix crash when internal port is removed.

Avoids a BSOD when AllowManagementOS is set to false.

Reported-at: https://github.com/openvswitch/ovs/issues/13
Reported-by: Alin Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Nithin Raju <nithin@vmware.com>