Fix divide by zero (bugs #2268 and 2259)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@3626 65c4cc65-6c06-0410-ace0-fbb531ad65f3

dsp.c

@@ -1228,6 +1228,9 @@ static int __ast_dsp_silence(struct ast_dsp *dsp, short *s, int len, int *totals
 	int accum;
 	int x;
 	int res = 0;
+
+	if (!len)
+		return 0;
 	
 	accum = 0;
 	for (x=0;x<len; x++)