ALSA: hda - Don't check capture source mixer if no ADC is available

With multiple codec configurations, some codec might have no ADC, thus
it keeps spec->adc_nids = NULL.  This causes an Oops in alc_build_controls().

Reference: kernel bug #16156
	https://bugzilla.kernel.org/show_bug.cgi?id=16156

Cc: <stable@kernel.org>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_realtek.c

@@ -2619,16 +2619,18 @@ static int alc_build_controls(struct hda_codec *codec)
 	}
 
 	/* assign Capture Source enums to NID */
-	kctl = snd_hda_find_mixer_ctl(codec, "Capture Source");
-	if (!kctl)
-		kctl = snd_hda_find_mixer_ctl(codec, "Input Source");
-	for (i = 0; kctl && i < kctl->count; i++) {
-		hda_nid_t *nids = spec->capsrc_nids;
-		if (!nids)
-			nids = spec->adc_nids;
-		err = snd_hda_add_nid(codec, kctl, i, nids[i]);
-		if (err < 0)
-			return err;
+	if (spec->capsrc_nids || spec->adc_nids) {
+		kctl = snd_hda_find_mixer_ctl(codec, "Capture Source");
+		if (!kctl)
+			kctl = snd_hda_find_mixer_ctl(codec, "Input Source");
+		for (i = 0; kctl && i < kctl->count; i++) {
+			hda_nid_t *nids = spec->capsrc_nids;
+			if (!nids)
+				nids = spec->adc_nids;
+			err = snd_hda_add_nid(codec, kctl, i, nids[i]);
+			if (err < 0)
+				return err;
+		}
 	}
 	if (spec->cap_mixer) {
 		const char *kname = kctl ? kctl->id.name : NULL;