HID: pidff: Fix null pointer dereference in pidff_find_fields

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

HID: pidff: Fix null pointer dereference in pidff_find_fields

This function triggered a null pointer dereference if used to search for

a report that isn't implemented on the device. This happened both for

optional and required reports alike.

The same logic was applied to pidff_find_special_field and although

pidff_init_fields should return an error earlier if one of the required

reports is missing, future modifications could change this logic and

resurface this possible null pointer dereference again.

LKML bug report:

https://lore.kernel.org/all/CAL-gK7f5=R0nrrQdPtaZZr1fd-cdAMbDMuZ_NLA8vM0SX+nGSw@mail.gmail.com