BUG FIXED: RoundRobinCounter.Add will cause a out of range crash

when index >= len(rrc.Values)

go/stats/duration_counter_test.go

@@ -0,0 +1,19 @@
+package stats
+
+import "testing"
+
+func TestRobinCounter(t *testing.T) {
+	rrc := NewRoundRobinCounter(60)
+	rrc.Add(0, 1)
+	rrc.Add(50, 2)
+	if rrc.Count() != 2 {
+		t.Fatal()
+	}
+	if rrc.Sum() != 3 {
+		t.Fatal()
+	}
+	/*
+		index out of range
+	*/
+	rrc.Add(61, 1)
+}