[ALSA] hda-codec - Add zero checks in input-mux helper functions

Added zero checks in input-mux helper functions to avoid Oops.
Some devices may have no input pins while the driver registers
control elements calling these functions.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>