ALSA: hda: fix some klockwork scan warnings

This patch fixes some warnings from klockwork.

These warnings are not the real issues. The patch
adds the sanity check.

Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/pci/hda/patch_hdmi.c b/sound/pci/hda/patch_hdmi.c
index d0d5ad8..56e5204 100644
--- a/sound/pci/hda/patch_hdmi.c
+++ b/sound/pci/hda/patch_hdmi.c
@@ -1680,6 +1680,11 @@
 
 	mutex_lock(&codec->spdif_mutex);
 	spdif = snd_hda_spdif_out_of_nid(codec, cvt_nid);
+	/* Add sanity check to pass klockwork check.
+	 * This should never happen.
+	 */
+	if (WARN_ON(spdif == NULL))
+		return true;
 	non_pcm = !!(spdif->status & IEC958_AES0_NONAUDIO);
 	mutex_unlock(&codec->spdif_mutex);
 	return non_pcm;