ALSA: hda: fix oopses in snd-hda-intel after digital slave support additions

Many places fail to check if codec has slave_dig_outs entries (the most common
case is not having any entry), leading to various possible oopses in hda_codec
code.

Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>