HID: check for NULL field when setting values

commit be67b68d upstream.

Defensively check that the field to be worked on is not NULL.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>