Bluetooth: Fix invalid-free in bcsp_close()

Syzbot reported an invalid-free that I introduced fixing a memleak.

bcsp_recv() also frees bcsp->rx_skb but never nullifies its value.
Nullify bcsp->rx_skb every time it is freed.

Signed-off-by: Tomas Bortoli <[e-mail]>
Reported-by: [e-mail]
Signed-off-by: Marcel Holtmann <[e-mail]>

drivers/bluetooth/hci_bcsp.c

···
 			if (*ptr == 0xc0) {
 				BT_ERR("Short BCSP packet");
 				kfree_skb(bcsp->rx_skb);
+				bcsp->rx_skb = NULL;
 				bcsp->rx_state = BCSP_W4_PKT_START;
 				bcsp->rx_count = 0;
 			} else
···
 			    bcsp->rx_skb->data[2])) != bcsp->rx_skb->data[3]) {
 				BT_ERR("Error in BCSP hdr checksum");
 				kfree_skb(bcsp->rx_skb);
+				bcsp->rx_skb = NULL;
 				bcsp->rx_state = BCSP_W4_PKT_DELIMITER;
 				bcsp->rx_count = 0;
 				continue;
···
 				       bscp_get_crc(bcsp));
 
 				kfree_skb(bcsp->rx_skb);
+				bcsp->rx_skb = NULL;
 				bcsp->rx_state = BCSP_W4_PKT_DELIMITER;
 				bcsp->rx_count = 0;
 				continue;