platform/x86: dell-wmi-descriptor: check if memory was allocated

devm_kzalloc will return NULL pointer if no memory was allocated.
This should be checked.  This problem also existed when the driver
was dell-wmi.c.

Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>