pgconn: check if pipeline i closed in Sync/GetResults

Otherwise there will be a nil pointer exception accessing the conn

pgconn/pgconn.go

@@ -2076,10 +2076,21 @@ 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
 	}
 
+	return p.getResults()
+}
+
+func (p *Pipeline) getResults() (results any, err error) {
 	for {
 		msg, err := p.conn.receiveMessage()
 		if err != nil {
@@ -2166,6 +2177,7 @@ func (p *Pipeline) Close() error {
 	if p.closed {
 		return p.err
 	}
+
 	p.closed = true
 
 	if p.pendingSync {
@@ -2178,7 +2190,7 @@ func (p *Pipeline) Close() error {
 	}
 
 	for p.expectedReadyForQueryCount > 0 {
-		_, err := p.GetResults()
+		_, err := p.getResults()
 		if err != nil {
 			p.err = err
 			var pgErr *PgError