taprio: add null check on sched_nest to avoid potential null pointer dereference

The call to nla_nest_start_noflag can return a null pointer and currently
this is not being checked and this can lead to a null pointer dereference
when the null pointer sched_nest is passed to function nla_nest_end. Fix
this by adding in a null pointer check.

Addresses-Coverity: ("Dereference null return value")
Fixes: a3d43c0d ("taprio: Add support adding an admin schedule")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>