ALSA: hda: dinput_mux check

Add check to determine if dinput_mux is set by any of patch_stac*() functions,
otherwise a invalid pointer my be referenced causing gibberish to mixer values.
Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_sigmatel.c

@@ -3589,7 +3589,8 @@ static int stac92xx_parse_auto_config(struct hda_codec *codec, hda_nid_t dig_out
 		spec->mixers[spec->num_mixers++] = spec->kctls.list;
 
 	spec->input_mux = &spec->private_imux;
-	spec->dinput_mux = &spec->private_dimux;
+	if (!spec->dinput_mux)
+		spec->dinput_mux = &spec->private_dimux;
 	spec->sinput_mux = &spec->private_smux;
 	spec->mono_mux = &spec->private_mono_mux;
 	spec->amp_mux = &spec->private_amp_mux;