ofproto-dpif-xlate: Fix confusion between "no stp port" and "stp port 0".
Commit
9d189a50e (ofproto-dpif-xlate: Pull STP xlation into
ofproto-dpif-xlate.) introduced the bug that considers 'stp_port_no'
of 0 as stp disabled on the port. However 'stp_port_no' is
actually the index of the stp struct's port array and ranges
between [0, STP_MAX_PORTS). So the bug allows the blocked
port keep transmitting packets and generates loop.
This commit fixes this bug.
Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>