sch_netem: fix a divide by zero in tabledist()

[ Upstream commit b41d936b5ecfdb3a4abc525ce6402a6c49cffddc ]

syzbot managed to crash the kernel in tabledist() loading
an empty distribution table.

	t = dist->table[rnd % dist->size];

Simply return an error when such load is attempted.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

net/sched/sch_netem.c

@@ -749,7 +749,7 @@ static int get_dist_table(struct Qdisc *sch, struct disttable **tbl,
 	struct disttable *d;
 	int i;
 
-	if (n > NETEM_DIST_MAX)
+	if (!n || n > NETEM_DIST_MAX)
 		return -EINVAL;
 
 	d = kvmalloc(sizeof(struct disttable) + n * sizeof(s16), GFP_KERNEL);