fix: reject portal based queries in pipeline mode instead of TypeError

[nigrosimone]

A portal stays open across more round trips. In pipeline mode the queries behind it are written without waiting, so the server answer them out of that portal: the rows go to the wrong query, the next read fails with portal "..." does not exist, and with pg-cursor the process dies with an uncaught TypeError: Cannot read properties of null (reading 'push') inside handleDataRow.

Two cases, both reproducible every time on a client with pipeline: true:

without cursor 1,2,3   (correct)
with cursor:   TypeError: Cannot read properties of null (reading 'push')
               at Cursor.handleDataRow (pg-cursor/index.js:116)