[ALSA] Fix possible invalid memory access in PCM core

snd_internval_list() may access invalid memory in the case count = 0
is given.  It shouldn't be passed, but it'd better to make the code
a bit more robust.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>