ALSA: hda/realtek - Fix Oops in alc_mux_select()

When no imux is available (e.g. a single capture source),
alc_auto_init_input_src() may trigger an Oops due to the access to -1.
Add a proper zero-check to avoid it.

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