Tx is nil bug!!

[zhushh]

@js-ojus

Hi, I have found that the following code is a bug:

	var tx *sql.Tx
	if otx == nil {
		tx, err := db.Begin()
		if err != nil {
			return 0, err
		}
		defer tx.Rollback()
	} else {
		tx = otx
	}

here tx, err := db.Begin() will make a temporary variable tx, so out of the if statement, tx also will be a nil value. Thus, when you call the interface which need parameter *sql.Tx and you give a nil to it, it will cause a panic. For example:

        gid, err := flow.Groups.New(nil, "nike", "G")

I give nil to Groups.New because I have called RegisterDB to make db is not null.

We can use this code to fix it:

    var tx *sql.Tx
    if otx == nil {
        var err error
        tx, err = db.Begin()
        if err != nil {
            return 0, err
        }
        defer tx.Rollback()
    } else {
        tx = otx
    }

[js-ojus]

@zhushh Ouch, ouch! Good catch! I fell into a pretty basic Go trap. :-(

I never discovered this because, in all my code, I created a top-level transaction that I passed in.

I shall correct this.