Fixed LP bug #794890.

Changed the code that processing of multi-updates and multi-deletes
with multitable views at the prepare stage.

A proper solution would be: never to perform any transformations of views
before and at the prepare stage. Yet it would  require re-engineering
of the code that checks privileges and updatability of views.
Ultimately this re-engineering has to be done to provide a clean solution
for INSERT/UPDATE/DELETE statements that use views.

Fixed a valgrind problem in the function TABLE::use_index.

sql/sql_class.h

@@ -1571,6 +1571,8 @@ public:
 
   bool prepare_derived_at_open;
 
+  bool save_prep_leaf_list;
+
 #ifndef MYSQL_CLIENT
   int binlog_setup_trx_data();