Bluetooth: hci_event: shut up a false-positive warning

Turning on -Wstringop-overflow globally exposed a misleading compiler
warning in bluetooth:

net/bluetooth/hci_event.c: In function 'hci_cc_read_class_of_dev':
net/bluetooth/hci_event.c:524:9: error: 'memcpy' writing 3 bytes into a
region of size 0 overflows the destination [-Werror=stringop-overflow=]
  524 |         memcpy(hdev->dev_class, rp->dev_class, 3);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The problem here is the check for hdev being NULL in bt_dev_dbg() that
leads the compiler to conclude that hdev->dev_class might be an invalid
pointer access.

Add another explicit check for the same condition to make sure gcc sees
this cannot happen.

Fixes: a9de9248 ("[Bluetooth] Switch from OGF+OCF to using only opcodes")
Fixes: 1b56c90018f0 ("Makefile: Enable -Wstringop-overflow globally")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/hci_event.c

@@ -516,6 +516,9 @@ static u8 hci_cc_read_class_of_dev(struct hci_dev *hdev, void *data,
 {
 	struct hci_rp_read_class_of_dev *rp = data;
 
+	if (WARN_ON(!hdev))
+		return HCI_ERROR_UNSPECIFIED;
+
 	bt_dev_dbg(hdev, "status 0x%2.2x", rp->status);
 
 	if (rp->status)