Fix a memory leak

callback_test.go

@@ -11,6 +11,7 @@ func TestCallback_001(t *testing.T) {
 	if reflect.TypeOf(v) != reflect.TypeOf(TestCallback_001) {
 		t.Error("Received unexpected value")
 	}
+	cbDelete(c)
 }
 
 func TestCallback_002(t *testing.T) {
@@ -21,3 +22,12 @@ func TestCallback_002(t *testing.T) {
 	cbGet(c + 1)
 	t.Error("Expected a panic")
 }
+
+func TestCallback_003(t *testing.T) {
+	defer func() {
+		recover()
+	}()
+	c := cbAdd(TestCallback_003)
+	cbDelete(c + 1)
+	t.Error("Expected a panic")
+}