dispatcher(k): fixed use of use_default parameter

- when there was a single destination in destination set and it was in
  inactive state, then having parameter use_default=1 caused a divion by
  0 while atteptimpting to look for active destaintaion
- reported by Eric Hiller

modules_k/dispatcher/dispatch.c

@@ -1717,15 +1717,20 @@ int ds_select_dst(struct sip_msg *msg, int set, int alg, int mode)
 	while ((idx->dlist[i].flags & DS_INACTIVE_DST)
 			|| (idx->dlist[i].flags & DS_PROBING_DST))
 	{
-		if(ds_use_default!=0)
+		if(ds_use_default!=0 && idx->nr!=1)
 			i = (i+1)%(idx->nr-1);
 		else
 			i = (i+1)%idx->nr;
 		if(i==hash)
 		{
+			/* back to start -- looks like no active dst */
 			if(ds_use_default!=0)
 			{
 				i = idx->nr-1;
+				if((idx->dlist[i].flags & DS_INACTIVE_DST)
+						|| (idx->dlist[i].flags & DS_PROBING_DST))
+					return -1;
+				break;
 			} else {
 				return -1;
 			}
@@ -1751,6 +1756,7 @@ int ds_select_dst(struct sip_msg *msg, int set, int alg, int mode)
 
 	if(dst_avp_name.n!=0)
 	{
+		/* add default dst to last position in AVP list */
 		if(ds_use_default!=0 && hash!=idx->nr-1)
 		{
 			avp_val.s = idx->dlist[idx->nr-1].uri;