relay_open() may return NULL, check the return value to avoid the crash.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
IP: [<ffffffffa01a95c5>] ath_cmn_process_fft+0xd5/0x700 [ath9k_common]
PGD 41cf28067 PUD 41be92067 PMD 0
Oops: 0000 [#1] SMP
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.8.6+ #35
Hardware name: Hewlett-Packard h8-1080t/2A86, BIOS 6.15    07/04/2011
task: ffffffff81e0c4c0 task.stack: ffffffff81e00000
RIP: 0010:[<ffffffffa01a95c5>] [<ffffffffa01a95c5>] ath_cmn_process_fft+0xd5/0x700 [ath9k_common]
RSP: 0018:ffff88041f203ca0 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 000000000000059f RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000040 RDI: ffffffff81f0ca98
RBP: ffff88041f203dc8 R08: ffffffffffffffff R09: 00000000000000ff
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffffffff81f0ca98 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff88041f200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000040 CR3: 000000041b6ec000 CR4: 00000000000006f0
Stack:
0000000000000363 00000000000003f3 00000000000003f3 00000000000001f9
000000000000049a 0000000001252c04 ffff88041f203e44 ffff880417b4bfd0
0000000000000008 ffff88041785b9c0 0000000000000002 ffff88041613dc60

Call Trace:
<IRQ>
[<ffffffffa01b6441>] ath9k_tasklet+0x1b1/0x220 [ath9k]
[<ffffffff8105d8dd>] tasklet_action+0x4d/0xf0
[<ffffffff8105dde2>] __do_softirq+0x92/0x2a0
Reported-by: Devin Tuchsen <devin.tuchsen@gmail.com>
Tested-by: Devin Tuchsen <devin.tuchsen@gmail.com>
Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>