hsr: fix a NULL pointer deref in hsr_dev_xmit()

hsr_port_get_hsr() could return NULL and kernel
could crash:

 BUG: kernel NULL pointer dereference, address: 0000000000000010
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 8000000074b84067 P4D 8000000074b84067 PUD 7057d067 PMD 0
 Oops: 0000 [#1] SMP PTI
 CPU: 0 PID: 754 Comm: a.out Not tainted 5.2.0-rc6+ #718
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-2.fc30 04/01/2014
 RIP: 0010:hsr_dev_xmit+0x20/0x31
 Code: 48 8b 1b eb e0 5b 5d 41 5c c3 66 66 66 66 90 55 48 89 fd 48 8d be 40 0b 00 00 be 04 00 00 00 e8 ee f2 ff ff 48 89 ef 48 89 c6 <48> 8b 40 10 48 89 45 10 e8 6c 1b 00 00 31 c0 5d c3 66 66 66 66 90
 RSP: 0018:ffffb5b400003c48 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: ffff9821b4509a88 RCX: 0000000000000000
 RDX: ffff9821b4509a88 RSI: 0000000000000000 RDI: ffff9821bc3fc7c0
 RBP: ffff9821bc3fc7c0 R08: 0000000000000000 R09: 00000000000c2019
 R10: 0000000000000000 R11: 0000000000000002 R12: ffff9821bc3fc7c0
 R13: ffff9821b4509a88 R14: 0000000000000000 R15: 000000000000006e
 FS:  00007fee112a1800(0000) GS:ffff9821bd800000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000010 CR3: 000000006e9ce000 CR4: 00000000000406f0
 Call Trace:
  <IRQ>
  netdev_start_xmit+0x1b/0x38
  dev_hard_start_xmit+0x121/0x21e
  ? validate_xmit_skb.isra.0+0x19/0x1e3
  __dev_queue_xmit+0x74c/0x823
  ? lockdep_hardirqs_on+0x12b/0x17d
  ip6_finish_output2+0x3d3/0x42c
  ? ip6_mtu+0x55/0x5c
  ? mld_sendpack+0x191/0x229
  mld_sendpack+0x191/0x229
  mld_ifc_timer_expire+0x1f7/0x230
  ? mld_dad_timer_expire+0x58/0x58
  call_timer_fn+0x12e/0x273
  __run_timers.part.0+0x174/0x1b5
  ? mld_dad_timer_expire+0x58/0x58
  ? sched_clock_cpu+0x10/0xad
  ? mark_lock+0x26/0x1f2
  ? __lock_is_held+0x40/0x71
  run_timer_softirq+0x26/0x48
  __do_softirq+0x1af/0x392
  irq_exit+0x53/0xa2
  smp_apic_timer_interrupt+0x1c4/0x1d9
  apic_timer_interrupt+0xf/0x20
  </IRQ>

Cc: Arvid Brodin <arvid.brodin@alten.se>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>