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

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

commit cce4aa378a049f4275416ee6302dd24f37b289df upstream.

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.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
[PG: in mainline, 21268961d3 rewrites and creates alc_mux_select, but the
code that needed the check still existed prior to that in alc_mux_enum_put]
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>