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

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>