platform/x86: dell-wmi-sysman: Fix WMI data block retrieval in sysfs callbacks

platform/x86: dell-wmi-sysman: Fix WMI data block retrieval in sysfs callbacks

After retrieving WMI data blocks in sysfs callbacks, check for the
validity of them before dereferencing their content.

Reported-by: Jan Graczyk <jangraczyk@yahoo.ca>
Closes: https://lore.kernel.org/r/CAHk-=wgMiSKXf7SvQrfEnxVtmT=QVQPjJdNjfm3aXS7wc=rzTw@mail.gmail.com/
Fixes: e8a60aa7404b ("platform/x86: Introduce support for Systems Management Driver over WMI for Dell Systems")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Armin Wolf <W_Armin@gmx.de>
Signed-off-by: Kurt Borja <kuurtb@gmail.com>
Link: https://lore.kernel.org/r/20250630-sysman-fix-v2-1-d185674d0a30@gmail.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>