Add select_send::cleanup.
Fix a compilation warning.
Issues spotted while working on the fix for Bug#12713.


sql-common/client.c:
  Fix a warning.
sql/sql_class.cc:
  Give a variable a more specific name. Rewrite an incorrect comment.
  Add a cleanup for select_send. The only case now this cleanup can be
  necessary is when we have a prepared statement inside a stored procedure, 
  and a continue handler. At first execution, the statement is killed
  after having executed select_send::send_fields. At the second execution
  it is killed after having executed select_send::send_fields.
sql/sql_class.h:
  Rename a member. Add comments.