HID: hid-lg4ff: Add check for empty lbuf

commit d180b649 upstream.

If an empty buf is received, lbuf is also empty. So lbuf is
accessed by index -1.

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

Fixes: f31a2de3

 ("HID: hid-lg4ff: Allow switching of Logitech gaming wheels between compatibility modes")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>