ALSA: usb - driver neglects kmalloc return value check and may deref NULL

sound/usb/pcm.c::snd_usb_pcm_check_knot() fails to check the return value
from kmalloc() and may end up dereferencing a null pointer.
The patch below (compile tested only) should take care of that little
problem.
Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -676,8 +676,10 @@ static int snd_usb_pcm_check_knot(struct snd_pcm_runtime *runtime,
 	if (!needs_knot)
 		return 0;
 
-	subs->rate_list.count = count;
 	subs->rate_list.list = kmalloc(sizeof(int) * count, GFP_KERNEL);
+	if (!subs->rate_list.list)
+		return -ENOMEM;
+	subs->rate_list.count = count;
 	subs->rate_list.mask = 0;
 	count = 0;
 	list_for_each_entry(fp, &subs->fmt_list, list) {