fixed to erase a warning: 'm_freeze: already set.'.

Previous version called freeze() in SQLFromParser::doJoineOneRow().
This applies freeze() for all rows (item groups). However, because
SQLTableElement::getActiveRow() returns rows (item group) that
have already been freezed. So warnings were displayed.

This patch moved the freeze() to SQLTableJoin::getActiveRow() where
the new item group is created.

This version passed all tests except for testMySQLWorker.