net: rfkill: gpio: Fix crash due to dereferencering uninitialized pointer

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

net: rfkill: gpio: Fix crash due to dereferencering uninitialized pointer

Since commit 7d5e9737efda ("net: rfkill: gpio: get the name and type from device property") rfkill_find_type() gets called with the possibly uninitialized "const char *type_name;" local variable.

On x86 systems when rfkill-gpio binds to a "BCM4752" or "LNV4752" acpi_device, the rfkill->type is set based on the ACPI acpi_device_id:

rfkill->type = (unsigned)id->driver_data;

and there is no "type" property so device_property_read_string() will fail and leave type_name uninitialized, leading to a potential crash.

rfkill_find_type() does accept a NULL pointer, fix the potential crash by initializing type_name to NULL.

Note likely sofar this has not been caught because:

1. Not many x86 machines actually have a "BCM4752"/"LNV4752" acpi_device
2. The stack happened to contain NULL where type_name is stored