HID: holtek: fix slab-out-of-bounds Write in holtek_kbd_input_event

There is a slab-out-of-bounds Write bug in hid-holtek-kbd driver.
The problem is the driver assumes the device must have an input
but some malicious devices violate this assumption.

Fix this by checking hid_device's input is non-empty before its usage.

Signed-off-by: Ma Ke <make_ruc2021@163.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>