The problem was that we did not handle errors properly in
JOIN::get_best_combination. In case an early error, JOIN->join_tab would
contain unintialized values, which would cause errors on cleanup().

The error in question was reported earlier, but not noticed until later.
One cause of this is that most of the sql_select.cc code just checks
thd->fatal_error and not thd->is_error().
Fixed by changing of checks of fatal_error to is_error().