ALSA: usb-audio: Fix NULL dererence at error path

At an error path to release URB buffers and contexts, the driver might
hit a NULL dererence for u->urb pointer, when u->buffer_size has been
already set but the actual URB allocation failed.

Fix it by adding the NULL check of urb.  Also, make sure that
buffer_size is cleared after the error path or the close.

Cc: <stable@vger.kernel.org>
Reported-by: Sabri N. Ferreiro <snferreiro1@gmail.com>
Link: https://lore.kernel.org/r/CAKG+3NRjTey+fFfUEGwuxL-pi_=T4cUskYG9OzpzHytF+tzYng@mail.gmail.com
Link: https://lore.kernel.org/r/20220930100129.19445-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/endpoint.c

@@ -95,12 +95,13 @@ static inline unsigned get_usb_high_speed_rate(unsigned int rate)
  */
 static void release_urb_ctx(struct snd_urb_ctx *u)
 {
-	if (u->buffer_size)
+	if (u->urb && u->buffer_size)
 		usb_free_coherent(u->ep->chip->dev, u->buffer_size,
 				  u->urb->transfer_buffer,
 				  u->urb->transfer_dma);
 	usb_free_urb(u->urb);
 	u->urb = NULL;
+	u->buffer_size = 0;
 }
 
 static const char *usb_error_string(int err)