Fixed valgrind warnings in delete_dynamic().

Join_plan_state performs out-of-API initialization of DYNAMIC_ARRAY. This is
done to postpone actual array initialization till first use, whilst retaining
the right to call delete_dynamic().

Since delete_dynamic() now checks DYNAMIC_ARRAY::malloc_flags it should be
initialized it as well.

sql/sql_select.h

@@ -903,6 +903,7 @@ protected:
     {   
       keyuse.elements= 0;
       keyuse.buffer= NULL;
+      keyuse.malloc_flags= 0;
       best_positions= 0;                        /* To detect errors */
       error= my_multi_malloc(MYF(MY_WME),
                              &best_positions,