sctp: fix CONFIG_SCTP_DBG_MSG=y null pointer dereference in sctp_v6_get_dst()

sctp: fix CONFIG_SCTP_DBG_MSG=y null pointer dereference in sctp_v6_get_dst()

Trinity (the syscall fuzzer) triggered the following BUG, reproducible
only when the kernel is configured with CONFIG_SCTP_DBG_MSG=y.

When CONFIG_SCTP_DBG_MSG is not set, the null pointer is never
dereferenced.

---[ end trace a4de0bfcb38a3642 ]---
BUG: unable to handle kernel NULL pointer dereference at 0000000000000100
IP: [<ffffffff8136796e>] ip6_string+0x1e/0xa0
PGD 4eead067 PUD 4e472067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in:
CPU 3
Pid: 21324, comm: trinity-child11 Tainted: G        W    3.7.0-rc7+ #61 ASUSTeK Computer INC. EB1012/EB1012
RIP: 0010:[<ffffffff8136796e>]  [<ffffffff8136796e>] ip6_string+0x1e/0xa0
RSP: 0018:ffff88004e4637a0  EFLAGS: 00010046
RAX: ffff88004e4637da RBX: ffff88004e4637da RCX: 0000000000000000
RDX: ffffffff8246e92a RSI: 0000000000000100 RDI: ffff88004e4637da
RBP: ffff88004e4637a8 R08: 000000000000ffff R09: 000000000000ffff
R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff8289d600
R13: ffffffff8289d230 R14: ffffffff8246e928 R15: ffffffff8289d600
FS:  00007fed95153700(0000) GS:ffff88005fd80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000100 CR3: 000000004eeac000 CR4: 00000000000007e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process trinity-child11 (pid: 21324, threadinfo ffff88004e462000, task ffff8800524b0000)
Stack:
 ffff88004e4637da ffff88004e463828 ffffffff81368eee 000000004e4637d8
 ffffffff0000ffff ffff88000000ffff 0000000000000000 000000004e4637f8
 ffffffff826285d8 ffff88004e4637f8 0000000000000000 ffff8800524b06b0
Call Trace:
 [<ffffffff81368eee>] ip6_addr_string.isra.11+0x3e/0xa0
 [<ffffffff81369183>] pointer.isra.12+0x233/0x2d0
 [<ffffffff810a413a>] ? vprintk_emit+0x1ba/0x450
 [<ffffffff8110953d>] ? trace_hardirqs_on_caller+0x10d/0x1a0
 [<ffffffff81369757>] vsnprintf+0x187/0x5d0
 [<ffffffff81369c62>] vscnprintf+0x12/0x30
 [<ffffffff810a4028>] vprintk_emit+0xa8/0x450
 [<ffffffff81e5cb00>] printk+0x49/0x4b
 [<ffffffff81d17221>] sctp_v6_get_dst+0x731/0x780
 [<ffffffff81d16e15>] ? sctp_v6_get_dst+0x325/0x780
 [<ffffffff81d00a96>] sctp_transport_route+0x46/0x120
 [<ffffffff81cff0f1>] sctp_assoc_add_peer+0x161/0x350
 [<ffffffff81d0fd8d>] sctp_sendmsg+0x6cd/0xcb0
 [<ffffffff81b55bf0>] ? inet_create+0x670/0x670
 [<ffffffff81b55cfb>] inet_sendmsg+0x10b/0x220
 [<ffffffff81b55bf0>] ? inet_create+0x670/0x670
 [<ffffffff81a72a64>] ? sock_update_classid+0xa4/0x2b0
 [<ffffffff81a72ab0>] ? sock_update_classid+0xf0/0x2b0
 [<ffffffff81a6ac1c>] sock_sendmsg+0xdc/0xf0
 [<ffffffff8118e9e5>] ? might_fault+0x85/0x90
 [<ffffffff8118e99c>] ? might_fault+0x3c/0x90
 [<ffffffff81a6e12a>] sys_sendto+0xfa/0x130
 [<ffffffff810a9887>] ? do_setitimer+0x197/0x380
 [<ffffffff81e960d5>] ? sysret_check+0x22/0x5d
 [<ffffffff81e960a9>] system_call_fastpath+0x16/0x1b
Code: 01 eb 89 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 f8 31 c9 48 89 e5 53 eb 12 0f 1f 40 00 48 83 c1 01 48 83 c0 04 48 83 f9 08 74 70 <0f> b6 3c 4e 89 fb 83 e7 0f c0 eb 04 41 89 d8 41 83 e0 0f 0f b6
RIP  [<ffffffff8136796e>] ip6_string+0x1e/0xa0
 RSP <ffff88004e4637a0>
CR2: 0000000000000100
---[ end trace a4de0bfcb38a3643 ]---

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>