net: ieee802154: nl-mac: fix check on panid

stable inclusion
from stable-5.10.30
commit 8bfb45fa131d90a8b9ab137b63d146658b196b82
bugzilla: 51791

--------------------------------

commit 6f7f657f upstream.

This patch fixes a null pointer derefence for panid handle by move the
check for the netlink variable directly before accessing them.

Reported-by:  <syzbot+d4c07de0144f6f63be3a@syzkaller.appspotmail.com>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20210228151817.95700-4-aahringo@redhat.com


Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by:   Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>