bugfix: multi-UPDATE, vcols, const tables

multi-update was setting up read_set/vcol_set in
multi_update::initialize_tables() that is invoked after
the optimizer (JOIN::optimize_inner()). But some rows - if they're from
const tables - will be read already in the optimizer, and these rows
will not have all necessary column/vcol values.

* multi_update::initialize_tables() uses results from the optimizer
  and cannot be moved to be called earlier.
* multi_update::prepare() is called before the optimizer, but
  it cannot set up read_set/vcol_set, because the optimizer
  might reset them (see SELECT_LEX::update_used_tables()).

As a fix I've added a new method, select_result::prepare_to_read_rows(),
it's called from inside the optimizer just before make_join_statistics().

sql/sql_select.cc

@@ -1466,6 +1466,7 @@ JOIN::optimize_inner()
   
   /* Calculate how to do the join */
   THD_STAGE_INFO(thd, stage_statistics);
+  result->prepare_to_read_rows();
   if (make_join_statistics(this, select_lex->leaf_tables, &keyuse) ||
       thd->is_fatal_error)
   {