Fix for Bug #9913 "udf_deinit is not called after execution of PS"

(aka "deinit is not called when calling udf from trigger").

We should call udf_deinit() function during cleanup phase after prepared
(or ordinary) statement execution instead of calling it from Item's
desctructor.

No test case is provided since it is hard to test UDF's from our test
suite.

sql/sql_udf.h

@@ -67,6 +67,7 @@ class udf_handler :public Sql_alloc
   bool get_arguments();
   bool fix_fields(THD *thd,struct st_table_list *tlist,Item_result_field *item,
 		  uint arg_count,Item **args);
+  void cleanup();
   double val(my_bool *null_value)
   {
     if (get_arguments())