iio: trigger: fix NULL pointer dereference in iio_trigger_write_current()

iio: trigger: fix NULL pointer dereference in iio_trigger_write_current()

In case oldtrig == trig == NULL (which happens when we set none
trigger, when there is already none set) there is a NULL pointer
dereference during iio_trigger_put(trig). Below is kernel output when
this occurs:

[   26.741790] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[   26.750179] pgd = cacc0000
[   26.752936] [00000000] *pgd=8adc6835, *pte=00000000, *ppte=00000000
[   26.759531] Internal error: Oops: 17 [#1] SMP ARM
[   26.764261] Modules linked in: usb_f_ncm u_ether usb_f_acm u_serial usb_f_fs libcomposite configfs evbug
[   26.773844] CPU: 0 PID: 152 Comm: synchro Not tainted 4.12.0-rc1 #2
[   26.780128] Hardware name: Freescale i.MX6 Ultralite (Device Tree)
[   26.786329] task: cb1de200 task.stack: cac92000
[   26.790892] PC is at iio_trigger_write_current+0x188/0x1f4
[   26.796403] LR is at lock_release+0xf8/0x20c
[   26.800696] pc : [<c0736f34>]    lr : [<c016efb0>]    psr: 600d0013
[   26.800696] sp : cac93e30  ip : cac93db0  fp : cac93e5c
[   26.812193] r10: c0e64fe8  r9 : 00000000  r8 : 00000001
[   26.817436] r7 : cb190810  r6 : 00000010  r5 : 00000001  r4 : 00000000
[   26.823982] r3 : 00000000  r2 : 00000000  r1 : cb1de200  r0 : 00000000
[   26.830528] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
[   26.837683] Control: 10c5387d  Table: 8acc006a  DAC: 00000051
[   26.843448] Process synchro (pid: 152, stack limit = 0xcac92210)
[   26.849475] Stack: (0xcac93e30 to 0xcac94000)
[   26.853857] 3e20:                                     00000001 c0736dac c054033c cae6b680
[   26.862060] 3e40: cae6b680 00000000 00000001 cb3f8610 cac93e74 cac93e60 c054035c c0736db8
[   26.870264] 3e60: 00000001 c054033c cac93e94 cac93e78 c029bf34 c0540348 00000000 00000000
[   26.878469] 3e80: cb3f8600 cae6b680 cac93ed4 cac93e98 c029b320 c029bef0 00000000 00000000
[   26.886672] 3ea0: 00000000 cac93f78 cb2d41fc caed3280 c029b214 cac93f78 00000001 000e20f8
[   26.894874] 3ec0: 00000001 00000000 cac93f44 cac93ed8 c0221dcc c029b220 c0e1ca39 cb2d41fc
[   26.903079] 3ee0: cac93f04 cac93ef0 c0183ef0 c0183ab0 cb2d41fc 00000000 cac93f44 cac93f08
[   26.911282] 3f00: c0225eec c0183ebc 00000001 00000000 c0223728 00000000 c0245454 00000001
[   26.919485] 3f20: 00000001 caed3280 000e20f8 cac93f78 000e20f8 00000001 cac93f74 cac93f48
[   26.927690] 3f40: c0223680 c0221da4 c0246520 c0245460 caed3283 caed3280 00000000 00000000
[   26.935893] 3f60: 000e20f8 00000001 cac93fa4 cac93f78 c0224520 c02235e4 00000000 00000000
[   26.944096] 3f80: 00000001 000e20f8 00000001 00000004 c0107f84 cac92000 00000000 cac93fa8
[   26.952299] 3fa0: c0107de0 c02244e8 00000001 000e20f8 0000000e 000e20f8 00000001 fbad2484
[   26.960502] 3fc0: 00000001 000e20f8 00000001 00000004 beb6b698 00064260 0006421c beb6b4b4
[   26.968705] 3fe0: 00000000 beb6b450 b6f219a0 b6e2f268 800d0010 0000000e cac93ff4 cac93ffc
[   26.976896] Backtrace:
[   26.979388] [<c0736dac>] (iio_trigger_write_current) from [<c054035c>] (dev_attr_store+0x20/0x2c)
[   26.988289]  r10:cb3f8610 r9:00000001 r8:00000000 r7:cae6b680 r6:cae6b680 r5:c054033c
[   26.996138]  r4:c0736dac r3:00000001
[   26.999747] [<c054033c>] (dev_attr_store) from [<c029bf34>] (sysfs_kf_write+0x50/0x54)
[   27.007686]  r5:c054033c r4:00000001
[   27.011290] [<c029bee4>] (sysfs_kf_write) from [<c029b320>] (kernfs_fop_write+0x10c/0x224)
[   27.019579]  r7:cae6b680 r6:cb3f8600 r5:00000000 r4:00000000
[   27.025271] [<c029b214>] (kernfs_fop_write) from [<c0221dcc>] (__vfs_write+0x34/0x120)
[   27.033214]  r10:00000000 r9:00000001 r8:000e20f8 r7:00000001 r6:cac93f78 r5:c029b214
[   27.041059]  r4:caed3280
[   27.043622] [<c0221d98>] (__vfs_write) from [<c0223680>] (vfs_write+0xa8/0x170)
[   27.050959]  r9:00000001 r8:000e20f8 r7:cac93f78 r6:000e20f8 r5:caed3280 r4:00000001
[   27.058731] [<c02235d8>] (vfs_write) from [<c0224520>] (SyS_write+0x44/0x98)
[   27.065806]  r9:00000001 r8:000e20f8 r7:00000000 r6:00000000 r5:caed3280 r4:caed3283
[   27.073582] [<c02244dc>] (SyS_write) from [<c0107de0>] (ret_fast_syscall+0x0/0x1c)
[   27.081179]  r9:cac92000 r8:c0107f84 r7:00000004 r6:00000001 r5:000e20f8 r4:00000001
[   27.088947] Code: 1a000009 e1a04009 e3a06010 e1a05008 (e5943000)
[   27.095244] ---[ end trace 06d1dab86d6e6bab ]---

To fix that problem call iio_trigger_put(trig) only when trig is not
NULL.

Fixes: d5d24bcc0a10 ("iio: trigger: close race condition in acquiring trigger reference")
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>