Bluetooth: Fix NULL pointer dereference when disconnecting

When disconnecting it is possible that the l2cap_conn pointer is already
NULL when bt_6lowpan_del_conn() is entered. Looking at l2cap_conn_del
also verifies this as there's a NULL check there too. This patch adds
the missing NULL check without which the following bug may occur:

BUG: unable to handle kernel NULL pointer dereference at   (null)
IP: [<c131e9c7>] bt_6lowpan_del_conn+0x19/0x12a
*pde = 00000000
Oops: 0000 [#1] SMP
CPU: 1 PID: 52 Comm: kworker/u5:1 Not tainted 3.12.0+ #196
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Workqueue: hci0 hci_rx_work
task: f6259b00 ti: f48c0000 task.ti: f48c0000
EIP: 0060:[<c131e9c7>] EFLAGS: 00010282 CPU: 1
EIP is at bt_6lowpan_del_conn+0x19/0x12a
EAX: 00000000 EBX: ef094e10 ECX: 00000000 EDX: 00000016
ESI: 00000000 EDI: f48c1e60 EBP: f48c1e50 ESP: f48c1e34
 DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
CR0: 8005003b CR2: 00000000 CR3: 30c65000 CR4: 00000690
Stack:
 f4d38000 00000000 f4d38000 00000002 ef094e10 00000016 f48c1e60 f48c1e70
 c1316bed f48c1e84 c1316bed 00000000 00000001 ef094e10 f48c1e84 f48c1ed0
 c1303cc6 c1303c7b f31f331a c1303cc6 f6e7d1c0 f3f8ea16 f3f8f380 f4d38008
Call Trace:
 [<c1316bed>] l2cap_disconn_cfm+0x3f/0x5b
 [<c1316bed>] ? l2cap_disconn_cfm+0x3f/0x5b
 [<c1303cc6>] hci_event_packet+0x645/0x2117
 [<c1303c7b>] ? hci_event_packet+0x5fa/0x2117
 [<c1303cc6>] ? hci_event_packet+0x645/0x2117
 [<c12681bd>] ? __kfree_skb+0x65/0x68
 [<c12681eb>] ? kfree_skb+0x2b/0x2e
 [<c130d3fb>] ? hci_send_to_sock+0x18d/0x199
 [<c12fa327>] hci_rx_work+0xf9/0x295
 [<c12fa327>] ? hci_rx_work+0xf9/0x295
 [<c1036d25>] process_one_work+0x128/0x1df
 [<c1346a39>] ? _raw_spin_unlock_irq+0x8/0x12
 [<c1036d25>] ? process_one_work+0x128/0x1df
 [<c103713a>] worker_thread+0x127/0x1c4
 [<c1037013>] ? rescuer_thread+0x216/0x216
 [<c103aec6>] kthread+0x88/0x8d
 [<c1040000>] ? task_rq_lock+0x37/0x6e
 [<c13474b7>] ret_from_kernel_thread+0x1b/0x28
 [<c103ae3e>] ? __kthread_parkme+0x50/0x50
Code: 05 b8 f4 ff ff ff 8d 65 f4 5b 5e 5f 5d 8d 67 f8 5f c3 57 8d 7c 24 08 83 e4 f8 ff 77 fc 55 89 e5 57 56f
EIP: [<c131e9c7>] bt_6lowpan_del_conn+0x19/0x12a SS:ESP 0068:f48c1e34
CR2: 0000000000000000
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/6lowpan.c

@@ -785,7 +785,7 @@ int bt_6lowpan_del_conn(struct l2cap_conn *conn)
 	unsigned long flags;
 	bool last = false;
 
-	if (!is_bt_6lowpan(conn->hcon))
+	if (!conn || !is_bt_6lowpan(conn->hcon))
 		return 0;
 
 	write_lock_irqsave(&devices_lock, flags);