With multiple codec configurations, some codec might have no ADC, thus
it keeps spec->adc_nids = NULL. This causes an Oops in alc_build_controls().
Reference: kernel bug #16156https://bugzilla.kernel.org/show_bug.cgi?id=16156
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>