BUG#787299: Valgrind complains on a join query with two IN subqueries

- Don't attempt to construct FirstMatch access method if we've 
  just figured three lines above that it can't be used (because join
  prefix doesn't have the needed tables), and so have set 
    pos->first_firstmatch_table= MAX_TABLES 
  Attempts to analyze join->positions[MAX_TABLES] caused valgrind warnings

sql/sql_select.h

@@ -584,6 +584,8 @@ typedef struct st_position
    */
   table_map firstmatch_need_tables;
 
+  bool in_firstmatch_prefix() { return (first_firstmatch_table != MAX_TABLES); }
+  void invalidate_firstmatch_prefix() { first_firstmatch_table= MAX_TABLES; }
 
 /* Duplicate Weedout strategy */
   /* The first table that the strategy will need to handle */