of_parse_thermal_zones() parses the thermal-zones node and registers a
thermal_zone device for each subnode. However, if a thermal zone is
consuming a thermal sensor and that thermal sensor device hasn't probed
yet, an attempt to set trip_point_*_temp for that thermal zone device
can cause a NULL pointer dereference. Fix it.

 console:/sys/class/thermal/thermal_zone87 # echo 120000 > trip_point_0_temp
 ...
 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000020
 ...
 Call trace:
  of_thermal_set_trip_temp+0x40/0xc4
  trip_point_temp_store+0xc0/0x1dc
  dev_attr_store+0x38/0x88
  sysfs_kf_write+0x64/0xc0
  kernfs_fop_write_iter+0x108/0x1d0
  vfs_write+0x2f4/0x368
  ksys_write+0x7c/0xec
  __arm64_sys_write+0x20/0x30
  el0_svc_common.llvm.7279915941325364641+0xbc/0x1bc
  do_el0_svc+0x28/0xa0
  el0_svc+0x14/0x24
  el0_sync_handler+0x88/0xec
  el0_sync+0x1c0/0x200

While at it, fix the possible NULL pointer dereference in other
functions as well: of_thermal_get_temp(), of_thermal_set_emul_temp(),
of_thermal_get_trend().
Suggested-by: David Collins <quic_collinsd@quicinc.com>
Signed-off-by: Subbaraman Narayanamurthy <quic_subbaram@quicinc.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>