Fix for bug lp:834492

Analysis:
In the test query semi-join merges the inner-most subquery
into the outer subquery, and the optimization of the merged
subquery finds some new index access methods. Later the
IN-EXISTS transformation is applied to the unmerged subquery.
Since the optimizer is instructed to not consider
materialization, it reoptimizes the plan in-place to take into
account the new IN-EXISTS conditions. Just before reoptimization
JOIN::choose_subquery_plan resets the query plan, which also
resets the access methods found during the semi-join merge.
Then reoptimization discovers there are no new access methods,
but it leaves the query plan in its reset state. Later semi-join
crashes because it assumes these access methods are present.

Solution:
When reoptimizing in-place, reset the query plan only after new
access methods were discovered. If no new access methods were
discovered, leave the current plan as it was.

sql/sql_select.h

@@ -698,6 +698,7 @@ protected:
   enum_reopt_result reoptimize(Item *added_where, table_map join_tables,
                                Join_plan_state *save_to);
   void save_query_plan(Join_plan_state *save_to);
+  void reset_query_plan();
   void restore_query_plan(Join_plan_state *restore_from);
   /* Choose a subquery plan for a table-less subquery. */
   bool choose_tableless_subquery_plan();