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.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>