platform/x86: dell-pc: avoid double free and invalid unregistration

platform/x86: dell-pc: avoid double free and invalid unregistration

If platform_profile_register() fails it does kfree(thermal_handler) and
leaves the pointer value around.
Any call to thermal_cleanup() will try to kfree(thermal_handler) again.
This will happen right away in dell_init().
In addition, platform_profile_remove() will be called although no
profile is registered.

NULL out the thermal_handler, so thermal_cleanup() avoids the double free.

Fixes: 996ad4129810 ("platform/x86: dell-pc: Implement platform_profile")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Lyndon Sanche <lsanche@lyndeno.ca>
Link: https://lore.kernel.org/r/20240604-dell-pc-double-free-v1-1-6d81255b2a44@weissschuh.net
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>