dpif-netdev: Fix another use-after-free in port_unref().
Commit
87400a3d4cc4a (dpif-netdev: Fix use-after-free in port_unref().)
fixed one use-after-free in the common case of port_unref(). However,
there was another, similar case: if port->netdev has no rxqs, then
the netdev_close() causes port->netdev to be destroyed and thus the
following call to netdev_n_rxq() accesses freed memory. This commit fixes
the problem.
Found by valgrind.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>