fix: free turn of connection pool when leave with error

[cyningsun]

pool turn is taken but without free after calling popIdle() and leave with error.

There are two if conditions may return error inside popIdle()

func (p *ConnPool) popIdle() (*Conn, error) {
	if p.closed() {
		return nil, ErrClosed
	}
	n := len(p.idleConns)
	if n == 0 {
		return nil, nil
	}