Get bound row in open(), in case fdbsql.pipeline.map.enabled is false.

The issue is that something inside the loop may use the same position. This is how the other operators work. Another possible fix would be to make binding positions more globally unique. But note that with fdbsql.pipeline.map.enabled=true, which will presumably be the default at some point, the bindings are a tree and no two operators share the same actual position even if they share the same index.

There is a failing test in the FTS, which wasn't running for a while, hence this going unnoticed when the operator was revised.