chan_iax2.c: Ensure all IEs are displayed when dumping frame contents.

When IAX2 debugging was enabled (`iax2 set debug on`), if the last IE
in a frame was one that may not have any data - such as the CALLTOKEN
IE in an NEW request - it was not getting displayed.

(cherry picked from commit 19507ae160)

channels/iax2/parser.c

@@ -424,7 +424,7 @@ static void dump_ies(unsigned char *iedata, int len)
 
 	if (len < 2)
 		return;
-	while(len > 2) {
+	while(len >= 2) {
 		ie = iedata[0];
 		ielen = iedata[1];
 		if (ielen + 2> len) {