beego: template fund when the start> len(bt)

@@ -27,6 +27,9 @@ func Substr(s string, start, length int) string {
 	if start < 0 {
 		start = 0
 	}
+	if start > len(bt) {
+		start = start % len(bt)
+	}
 	var end int
 	if (start + length) > (len(bt) - 1) {
 		end = len(bt)

@@ -25,6 +25,9 @@ func TestSubstr(t *testing.T) {
 	if Substr(s, 0, 100) != "012345" {
 		t.Error("should be equal")
 	}
+	if Substr(s, 12, 100) != "012345" {
+		t.Error("should be equal")
+	}
 }
 
 func TestHtml2str(t *testing.T) {