HID: pidff: Fix possible null pointer dereference

HID: pidff: Fix possible null pointer dereference

As reported by Dan Carpenter, if the axes_enable field wasn't found,
trying to find the axes themselves will result in a null pointer
dereference. This could only occur with a broken PID descriptor, but
it's worth protecting from.

Exit early if the axes_enable wasn't found AND add a gate to the
pidff_find_special_keys to exit early if the passed HID field is null.
This will protect again null dereferencing in the future and properly
return 0 found special keys.

Fixes: 1d72e7bd340b ("HID: pidff: Add support for AXES_ENABLE field")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Tomasz Pakuła <tomasz.pakula.oficjalny@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>