Fix for Bug#5034 "prepared "select 1 into @arg15", second

execute crashes server": we were deleting lex->result
after each execute, but prepared statements assumed that
it's left intact.
The fix adds cleanup() method to select_result hierarchy,
so that result objects can be reused.
Plus we now need to delete result objects more wisely.

sql/sql_insert.cc

@@ -1465,6 +1465,13 @@ select_insert::prepare(List<Item> &values, SELECT_LEX_UNIT *u)
   DBUG_RETURN(0);
 }
 
+
+void select_insert::cleanup()
+{
+  /* select_insert/select_create are never re-used in prepared statement */
+  DBUG_ASSERT(0);
+}
+
 select_insert::~select_insert()
 {
   if (table)