drivers/hid/hid-multitouch.c: fix a possible null pointer access.

1002     if ((quirks & MT_QUIRK_IGNORE_DUPLICATES) && mt) {
1003         struct input_mt_slot *i_slot = &mt->slots[slotnum];
1004
1005         if (input_mt_is_active(i_slot) &&
1006             input_mt_is_used(mt, i_slot))
1007             return -EAGAIN;
1008     }

We previously assumed 'mt' could be null (see line 1002).

The following situation is similar, so add a judgement.

Signed-off-by: Pan Zhang <zhangpan26@huawei.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>