Bluetooth: fixing null-pointer dereference in l2cap channel destroyer

During local testing with UBSAN enabled, warning was reported:
bluetooth/host/l2cap.c:980:25: runtime error: member access
within null pointer of type 'struct k_work_q'

It turned out that le_chan->rtx_work.queue can be NULL.
Since null-pointer dereference is a UB, additional check
was added to ensure we don't access
`le_chan->rtx_work.queue->thread` when
`le_chan->rtx_work.queue == NULL`

The same changes applied to l2cap_br.c

Signed-off-by: Ivan Iushkov <ivan.iushkov@nordicsemi.no>

subsys/bluetooth/host/l2cap.c

@@ -962,7 +962,9 @@ static void l2cap_chan_destroy(struct bt_l2cap_chan *chan)
 	 * In the case where we are in the context of executing the rtx_work
 	 * item, we don't sync as it will deadlock the workqueue.
 	 */
-	if (k_current_get() != &le_chan->rtx_work.queue->thread) {
+	struct k_work_q *rtx_work_queue = le_chan->rtx_work.queue;
+
+	if (rtx_work_queue == NULL || k_current_get() != &rtx_work_queue->thread) {
 		k_work_cancel_delayable_sync(&le_chan->rtx_work, &le_chan->rtx_sync);
 	} else {
 		k_work_cancel_delayable(&le_chan->rtx_work);

subsys/bluetooth/host/l2cap_br.c

@@ -165,7 +165,9 @@ static void l2cap_br_chan_destroy(struct bt_l2cap_chan *chan)
 	 * In the case where we are in the context of executing the rtx_work
 	 * item, we don't sync as it will deadlock the workqueue.
 	 */
-	if (k_current_get() != &br_chan->rtx_work.queue->thread) {
+	struct k_work_q *rtx_work_queue = br_chan->rtx_work.queue;
+
+	if (rtx_work_queue == NULL || k_current_get() != &rtx_work_queue->thread) {
 		k_work_cancel_delayable_sync(&br_chan->rtx_work, &br_chan->rtx_sync);
 	} else {
 		k_work_cancel_delayable(&br_chan->rtx_work);