zebra: Prevent crash because nl is NULL on shutdown

zebra: Prevent crash because nl is NULL on shutdown

When shutting down the main pthread was first closing
the sockets associated with the dplane pthread and
then telling it to shutdown the pthread at a later point
in time.  This caused the dplane to crash because the nl
data has been freed already.  Change the shutdown order
to stop the dplane pthread *and* then close the sockets.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>