ALSA: usb-audio: Fix possible NULL pointer dereference in snd_usb_pcm_has_fixed_rate()

The subs function argument may be NULL, so do not use it before the NULL check.

Fixes: 291e9da914 ("ALSA: usb-audio: Always initialize fixed_rate in snd_usb_find_implicit_fb_sync_format()")
Reported-by: coverity-bot <keescook@chromium.org>
Link: https://lore.kernel.org/alsa-devel/202301121424.4A79A485@keescook/
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230113085311.623325-1-perex@perex.cz
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -160,11 +160,12 @@ find_substream_format(struct snd_usb_substream *subs,
 bool snd_usb_pcm_has_fixed_rate(struct snd_usb_substream *subs)
 {
 	const struct audioformat *fp;
-	struct snd_usb_audio *chip = subs->stream->chip;
+	struct snd_usb_audio *chip;
 	int rate = -1;
 
 	if (!subs)
 		return false;
+	chip = subs->stream->chip;
 	if (!(chip->quirk_flags & QUIRK_FLAG_FIXED_RATE))
 		return false;
 	list_for_each_entry(fp, &subs->fmt_list, list) {