Don't query a widget parameter for an invalid NID in get_wcaps() but
rather returns zero (i.e. no attribute).
The read to an non-existing widget may result in a fatal codec
communication error.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@perex.cz>