thermal: core: Fix NULL pointer dereference in zone registration error path

In the Linux kernel, the following vulnerability has been resolved:

thermal: core: Fix NULL pointer dereference in zone registration error path

If device_register() in thermal_zone_device_register_with_trips() returns an error, the tz variable is set to NULL and subsequently dereferenced in kfree(tz->tzp).

Commit adc8749b150c ("thermal/drivers/core: Use put_device() if device_register() fails") added the tz = NULL assignment in question to avoid a possible double-free after dropping the reference to the zone device. However, after commit 4649620d9404 ("thermal: core: Make thermal_zone_device_unregister() return after freeing the zone"), that assignment has become redundant, because dropping the reference to the zone device does not cause the zone object to be freed any more.

Drop it to address the NULL pointer dereference.