Thermal: fix a NULL pointer dereference when generic thermal layer is built as a module

[   12.761956] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
[   12.762016] IP: [<ffffffffa0005277>] handle_thermal_trip+0x47/0x130 [thermal_sys]
[   12.762060] PGD 1fec74067 PUD 1fee5b067 PMD 0
[   12.762127] Oops: 0000 [#1] SMP
[   12.762177] Modules linked in: hid_generic crc32c_intel usbhid hid firewire_ohci(+) e1000e(+) firewire_core crc_itu_t xhci_hcd(+) thermal(+) fan thermal_sys hwmon
[   12.762423] CPU 1
[   12.762443] Pid: 187, comm: modprobe Tainted: G       A     3.7.0-thermal-module+ #25                  /DH77DF
[   12.762496] RIP: 0010:[<ffffffffa0005277>]  [<ffffffffa0005277>] handle_thermal_trip+0x47/0x130 [thermal_sys]
[   12.762682] RSP: 0018:ffff8801fe7ddc18  EFLAGS: 00010282
[   12.762704] RAX: 0000000000000000 RBX: ffff8801ff3e9c00 RCX: ffff8801fdc39800
[   12.762728] RDX: ffff8801fe7ddc24 RSI: 0000000000000001 RDI: ffff8801ff3e9c00
[   12.762764] RBP: ffff8801fe7ddc48 R08: 0000000004000000 R09: ffffffffa001f568
[   12.762797] R10: ffffffff81363083 R11: 0000000000000001 R12: 0000000000000001
[   12.762832] R13: 0000000000000000 R14: 0000000000000001 R15: ffff8801fde73e68
[   12.762866] FS:  00007f5548516700(0000) GS:ffff88021f240000(0000) knlGS:0000000000000000
[   12.762912] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   12.762946] CR2: 0000000000000018 CR3: 00000001fefe2000 CR4: 00000000001407e0
[   12.762979] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   12.763014] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   12.763048] Process modprobe (pid: 187, threadinfo ffff8801fe7dc000, task ffff8801fe5bdb40)
[   12.763095] Stack:
[   12.763122]  0000000000019640 00000000fdc39800 ffff8801fe7ddc48 ffff8801ff3e9c00
[   12.763225]  0000000000000002 0000000000000000 ffff8801fe7ddc78 ffffffffa00053e7
[   12.763338]  ffff8801ff3e9c00 0000000000006c98 ffffffffa0007480 ffff8801ff3e9c00
[   12.763440] Call Trace:
[   12.763470]  [<ffffffffa00053e7>] thermal_zone_device_update+0x77/0xa0 [thermal_sys]
[   12.763515]  [<ffffffffa0006d38>] thermal_zone_device_register+0x788/0xa88 [thermal_sys]
[   12.763562]  [<ffffffffa001f394>] acpi_thermal_add+0x360/0x4c8 [thermal]
[   12.763598]  [<ffffffff8133902a>] acpi_device_probe+0x50/0x190
[   12.763632]  [<ffffffff811bd793>] ? sysfs_create_link+0x13/0x20
[   12.763666]  [<ffffffff813cc41b>] driver_probe_device+0x7b/0x240
[   12.763699]  [<ffffffff813cc68b>] __driver_attach+0xab/0xb0
[   12.763732]  [<ffffffff813cc5e0>] ? driver_probe_device+0x240/0x240
[   12.763766]  [<ffffffff813ca836>] bus_for_each_dev+0x56/0x90
[   12.763799]  [<ffffffff813cbf4e>] driver_attach+0x1e/0x20
[   12.763831]  [<ffffffff813cbac0>] bus_add_driver+0x190/0x290
[   12.763864]  [<ffffffffa0022000>] ? 0xffffffffa0021fff
[   12.763896]  [<ffffffff813ccbea>] driver_register+0x7a/0x160
[   12.763928]  [<ffffffffa0022000>] ? 0xffffffffa0021fff
[   12.763960]  [<ffffffff813399fb>] acpi_bus_register_driver+0x43/0x45
[   12.763995]  [<ffffffffa002203a>] acpi_thermal_init+0x3a/0x42 [thermal]
[   12.764029]  [<ffffffff8100207f>] do_one_initcall+0x3f/0x170
[   12.764063]  [<ffffffff810b1a5f>] sys_init_module+0x8f/0x200
[   12.764097]  [<ffffffff815ff259>] system_call_fastpath+0x16/0x1b
[   12.764129] Code: 48 8b 87 c8 02 00 00 41 89 f4 48 8d 55 dc ff 50 28 44 8b 6d dc 41 8d 45 fe 83 f8 01 76 5e 48 8b 83 d8 02 00 00 44 89 e6 48 89 df <ff> 50 18 4c 8d a3 10 03 00 00 4c 89 e7 e8 87 f1 5e e1 8b 83 bc
[   12.765164] RIP  [<ffffffffa0005277>] handle_thermal_trip+0x47/0x130 [thermal_sys]
[   12.765223]  RSP <ffff8801fe7ddc18>
[   12.765252] CR2: 0000000000000018
[   12.765284] ---[ end trace 7723294cdfb00d2a ]---

This is because thermal_zone_device_update() is invoked before
any thermal governors being registered.
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

drivers/thermal/thermal_sys.c

@@ -336,7 +336,8 @@ static void monitor_thermal_zone(struct thermal_zone_device *tz)
 static void handle_non_critical_trips(struct thermal_zone_device *tz,
 			int trip, enum thermal_trip_type trip_type)
 {
-	tz->governor->throttle(tz, trip);
+	if (tz->governor)
+		tz->governor->throttle(tz, trip);
 }
 
 static void handle_critical_trips(struct thermal_zone_device *tz,