platform/x86: dell-wmi-sysman: Avoid buffer overflow in current_password_store()

platform/x86: dell-wmi-sysman: Avoid buffer overflow in current_password_store()

If the 'buf' array received from the user contains an empty string, the
'length' variable will be zero. Accessing the 'buf' array element with
index 'length - 1' will result in a buffer overflow.

Add a check for an empty string.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: e8a60aa7404b ("platform/x86: Introduce support for Systems Management Driver over WMI for Dell Systems")
Cc: stable@vger.kernel.org
Signed-off-by: Vladimir Moskovkin <Vladimir.Moskovkin@kaspersky.com>
Link: https://lore.kernel.org/r/39973642a4f24295b4a8fad9109c5b08@kaspersky.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>