Bluetooth: 6lowpan: check skb_clone() return value in send_mcast_pkt()

Bluetooth: 6lowpan: check skb_clone() return value in send_mcast_pkt()

The skb_clone() function can return NULL if memory allocation fails.
send_mcast_pkt() calls skb_clone() without checking the return value, which
can lead to a NULL pointer dereference in send_pkt() when it dereferences
skb->data.
Add a NULL check after skb_clone() and skip the peer if the clone fails.

Fixes: 18722c247023 ("Bluetooth: Enable 6LoWPAN support for BT LE devices")
Signed-off-by: Zhao Dongdong <zhaodongdong@kylinos.cn>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>