ALSA: usb-audio: Fix missing error check at mixer resolution test

A check of the return value from get_cur_mix_raw() is missing at the
resolution test code in get_min_max_with_quirks(), which may leave the
variable untouched, leading to a random uninitialized value, as
detected by syzkaller fuzzer.

Add the missing return error check for fixing that.

Reported-and-tested-by: syzbot+abe1ab7afc62c6bb6377@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191109181658.30368-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -1229,7 +1229,8 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
 		if (cval->min + cval->res < cval->max) {
 			int last_valid_res = cval->res;
 			int saved, test, check;
-			get_cur_mix_raw(cval, minchn, &saved);
+			if (get_cur_mix_raw(cval, minchn, &saved) < 0)
+				goto no_res_check;
 			for (;;) {
 				test = saved;
 				if (test < cval->max)
@@ -1249,6 +1250,7 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
 			snd_usb_set_cur_mix_value(cval, minchn, 0, saved);
 		}
 
+no_res_check:
 		cval->initialized = 1;
 	}