ALSA: usb-audio: Fix NULL pointer deference in try_to_register_card

In try_to_register_card(), the return value of usb_ifnum_to_if() is
passed directly to usb_interface_claimed() without a NULL check, which
will lead to a NULL pointer dereference when creating an invalid
USB audio device. Fix this by adding a check to ensure the interface
pointer is valid before passing it to usb_interface_claimed().

Fixes: 39efc9c8a973 ("ALSA: usb-audio: Fix last interface check for registration")
Closes: https://lore.kernel.org/all/CANypQFYtQxHL5ghREs-BujZG413RPJGnO5TH=[e-mail]/
Signed-off-by: Jiaming Zhang <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

sound/usb/card.c

···
  */
 static int try_to_register_card(struct snd_usb_audio *chip, int ifnum)
 {
+	struct usb_interface *iface;
+
 	if (check_delayed_register_option(chip) == ifnum ||
-	    chip->last_iface == ifnum ||
-	    usb_interface_claimed(usb_ifnum_to_if(chip->dev, chip->last_iface)))
+	    chip->last_iface == ifnum)
 		return snd_card_register(chip->card);
+
+	iface = usb_ifnum_to_if(chip->dev, chip->last_iface);
+	if (iface && usb_interface_claimed(iface))
+		return snd_card_register(chip->card);
+
 	return 0;
 }