ALSA: hda - Fix NULL-derefence with a single mic in STAC auto-mic detection

When only one mic is available and it's an analog mic, the current
IDT/STAC parser may give an Oops.

Reference: bko#25692
	https://bugzilla.kernel.org/show_bug.cgi?id=25692

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

sound/pci/hda/patch_sigmatel.c

@@ -3594,7 +3594,7 @@ static int stac_check_auto_mic(struct hda_codec *codec)
 		if (check_mic_pin(codec, spec->dmic_nids[i],
 		    &fixed, &ext, &dock))
 			return 0;
-	if (!fixed && !ext && !dock)
+	if (!fixed || (!ext && !dock))
 		return 0; /* no input to switch */
 	if (!(get_wcaps(codec, ext) & AC_WCAP_UNSOL_CAP))
 		return 0; /* no unsol support */