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 <tomasbortoli@gmail.com>
Reported-by: syzbot+a0d209a4676664613e76@syzkaller.appspotmail.com
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/hci_bcsp.c

@@ -591,6 +591,7 @@ static int bcsp_recv(struct hci_uart *hu, const void *data, int count)
 			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
@@ -606,6 +607,7 @@ static int bcsp_recv(struct hci_uart *hu, const void *data, int count)
 			    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;
@@ -630,6 +632,7 @@ static int bcsp_recv(struct hci_uart *hu, const void *data, int count)
 				       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;