pgconn: check if pipeline i closed in Sync/GetResults

Otherwise there will be a nil pointer exception accessing the conn

pgconn/pgconn.go

@@ -2053,6 +2053,13 @@ func (p *Pipeline) Flush() error {
 
 // Sync establishes a synchronization point and flushes the queued requests.
 func (p *Pipeline) Sync() error {
+	if p.closed {
+		if p.err != nil {
+			return p.err
+		}
+		return errors.New("pipeline closed")
+	}
+
 	p.conn.frontend.SendSync(&pgproto3.Sync{})
 	err := p.Flush()
 	if err != nil {
@@ -2069,6 +2076,13 @@ func (p *Pipeline) Sync() error {
 // *PipelineSync. If an ErrorResponse is received from the server, results will be nil and err will be a *PgError. If no
 // results are available, results and err will both be nil.
 func (p *Pipeline) GetResults() (results any, err error) {
+	if p.closed {
+		if p.err != nil {
+			return nil, p.err
+		}
+		return nil, errors.New("pipeline closed")
+	}
+
 	if p.expectedReadyForQueryCount == 0 {
 		return nil, nil
 	}