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.