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>