dpif-netdev: Fix use-after-free in port_unref().

When the last rxq is closed (which releases the rxq's internal reference
to its netdev) the next call to netdev_n_rxq() accesses freed memory.

Found by valgrind.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Reported-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
dpif-netdev: Fix memory leak.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>