ALSA: hda - fix NULL-dereference in patch_realtek

Fix NULL-dereference when try to use alt_playback since those codecs
which support multistreaming playback usually have more than 1 adc but
the driver should create alt_capture when spec->stream_analog_alt_capture
is also defined.
Signed-off-by: NRaymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_realtek.c

@@ -4424,7 +4424,7 @@ static int alc_build_pcms(struct hda_codec *codec)
 				alc_pcm_null_stream;
 			info->stream[SNDRV_PCM_STREAM_PLAYBACK].nid = 0;
 		}
-		if (spec->num_adc_nids > 1) {
+		if (spec->num_adc_nids > 1 && spec->stream_analog_alt_capture) {
 			info->stream[SNDRV_PCM_STREAM_CAPTURE] =
 				*spec->stream_analog_alt_capture;
 			info->stream[SNDRV_PCM_STREAM_CAPTURE].nid =