Bluetooth: hci_uart: fix GPF in h5_recv

Syzbot hit general protection fault in h5_recv(). The problem was in
missing NULL check.

hu->serdev can be NULL and we cannot blindly pass &serdev->dev
somewhere, since it can cause GPF.

Fixes: d9dd833c ("Bluetooth: hci_h5: Add runtime suspend")
Reported-and-tested-by: syzbot+7d41312fe3f123a6f605@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/hci_h5.c

@@ -587,9 +587,11 @@ static int h5_recv(struct hci_uart *hu, const void *data, int count)
 		count -= processed;
 	}
 
-	pm_runtime_get(&hu->serdev->dev);
-	pm_runtime_mark_last_busy(&hu->serdev->dev);
-	pm_runtime_put_autosuspend(&hu->serdev->dev);
+	if (hu->serdev) {
+		pm_runtime_get(&hu->serdev->dev);
+		pm_runtime_mark_last_busy(&hu->serdev->dev);
+		pm_runtime_put_autosuspend(&hu->serdev->dev);
+	}
 
 	return 0;
 }