[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>

sound/core/pcm_lib.c

@@ -781,6 +781,11 @@ int snd_interval_list(struct snd_interval *i, unsigned int count, unsigned int *
 {
         unsigned int k;
 	int changed = 0;
+
+	if (!count) {
+		i->empty = 1;
+		return -EINVAL;
+	}
         for (k = 0; k < count; k++) {
 		if (mask && !(mask & (1 << k)))
 			continue;