hsr: fix a NULL pointer dereference in hsr_dev_xmit()

hsr: fix a NULL pointer dereference in hsr_dev_xmit()

hsr_dev_xmit() calls hsr_port_get_hsr() to find master node and that would
return NULL if master node is not existing in the list.
But hsr_dev_xmit() doesn't check return pointer so a NULL dereference
could occur.

Test commands:
    ip netns add nst
    ip link add veth0 type veth peer name veth1
    ip link add veth2 type veth peer name veth3
    ip link set veth1 netns nst
    ip link set veth3 netns nst
    ip link set veth0 up
    ip link set veth2 up
    ip link add hsr0 type hsr slave1 veth0 slave2 veth2
    ip a a 192.168.100.1/24 dev hsr0
    ip link set hsr0 up
    ip netns exec nst ip link set veth1 up
    ip netns exec nst ip link set veth3 up
    ip netns exec nst ip link add hsr1 type hsr slave1 veth1 slave2 veth3
    ip netns exec nst ip a a 192.168.100.2/24 dev hsr1
    ip netns exec nst ip link set hsr1 up
    hping3 192.168.100.2 -2 --flood &
    modprobe -rv hsr

Splat looks like:
[  217.351122][ T1635] kasan: CONFIG_KASAN_INLINE enabled
[  217.352969][ T1635] kasan: GPF could be caused by NULL-ptr deref or user memory access
[  217.354297][ T1635] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  217.355507][ T1635] CPU: 1 PID: 1635 Comm: hping3 Not tainted 5.4.0+ #192
[  217.356472][ T1635] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[  217.357804][ T1635] RIP: 0010:hsr_dev_xmit+0x34/0x90 [hsr]
[  217.373010][ T1635] Code: 48 8d be 00 0c 00 00 be 04 00 00 00 48 83 ec 08 e8 21 be ff ff 48 8d 78 10 48 ba 00 b
[  217.376919][ T1635] RSP: 0018:ffff8880cd8af058 EFLAGS: 00010202
[  217.377571][ T1635] RAX: 0000000000000000 RBX: ffff8880acde6840 RCX: 0000000000000002
[  217.379465][ T1635] RDX: dffffc0000000000 RSI: 0000000000000004 RDI: 0000000000000010
[  217.380274][ T1635] RBP: ffff8880acde6840 R08: ffffed101b440d5d R09: 0000000000000001
[  217.381078][ T1635] R10: 0000000000000001 R11: ffffed101b440d5c R12: ffff8880bffcc000
[  217.382023][ T1635] R13: ffff8880bffcc088 R14: 0000000000000000 R15: ffff8880ca675c00
[  217.383094][ T1635] FS:  00007f060d9d1740(0000) GS:ffff8880da000000(0000) knlGS:0000000000000000
[  217.384289][ T1635] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  217.385009][ T1635] CR2: 00007faf15381dd0 CR3: 00000000d523c001 CR4: 00000000000606e0
[  217.385940][ T1635] Call Trace:
[  217.386544][ T1635]  dev_hard_start_xmit+0x160/0x740
[  217.387114][ T1635]  __dev_queue_xmit+0x1961/0x2e10
[  217.388118][ T1635]  ? check_object+0xaf/0x260
[  217.391466][ T1635]  ? __alloc_skb+0xb9/0x500
[  217.392017][ T1635]  ? init_object+0x6b/0x80
[  217.392629][ T1635]  ? netdev_core_pick_tx+0x2e0/0x2e0
[  217.393175][ T1635]  ? __alloc_skb+0xb9/0x500
[  217.393727][ T1635]  ? rcu_read_lock_sched_held+0x90/0xc0
[  217.394331][ T1635]  ? rcu_read_lock_bh_held+0xa0/0xa0
[  217.395013][ T1635]  ? kasan_unpoison_shadow+0x30/0x40
[  217.395668][ T1635]  ? __kasan_kmalloc.constprop.4+0xa0/0xd0
[  217.396280][ T1635]  ? __kmalloc_node_track_caller+0x3a8/0x3f0
[  217.399007][ T1635]  ? __kasan_kmalloc.constprop.4+0xa0/0xd0
[  217.400093][ T1635]  ? __kmalloc_reserve.isra.46+0x2e/0xb0
[  217.401118][ T1635]  ? memset+0x1f/0x40
[  217.402529][ T1635]  ? __alloc_skb+0x317/0x500
[  217.404915][ T1635]  ? arp_xmit+0xca/0x2c0
[ ... ]

Fixes: 311633b60406 ("hsr: switch ->dellink() to ->ndo_uninit()")
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>