openvswitch: check for null pointer return from nla_nest_start_noflag

The call to nla_nest_start_noflag can return null in the unlikely
event that nla_put returns -EMSGSIZE.  Check for this condition to
avoid a null pointer dereference on pointer nla_reply.

Addresses-Coverity: ("Dereference null return value")
Fixes: 11efd5cb ("openvswitch: Support conntrack zone limit")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>