prepared statements with cursor protocol).

The problem was a bug in Materialized-cursor implementation.
Materialized_cursor::open() called send_result_metadata()
with items pointing to already closed table.

The fix is to send metadata when the table is still open.

NOTE: this is a "partial" fix: metadata are different with
and without --cursor-protocol, but that's a different large
problem, one indication of which is reported as Bug 24176.