Fix a valgrind warning.

sql/sql_prepare.cc:
  A small fix for the previous patch: we should first free the
  prepared statement items, and then free the runtime memory root,
  as some memory used for cleanup is allocated in that mem root.
sql/sql_select.cc:
  - ever free the cursor mem root in close() (it's too early).

sql/sql_prepare.cc

@@ -2447,6 +2447,7 @@ Prepared_statement::~Prepared_statement()
   if (cursor)
     cursor->Cursor::~Cursor();
   free_items();
+  free_root(cursor->mem_root, MYF(0));
   delete lex->result;
 }
 

sql/sql_select.cc

@@ -1920,7 +1920,6 @@ Cursor::~Cursor()
 {
   if (is_open())
     close(FALSE);
-  free_root(mem_root, MYF(0));
 }
 
 /*********************************************************************/