Bug#12713 "Error in a stored function called from a SELECT doesn't

cause ROLLBACK of statement", part 1. Review fixes.

Do not send OK/EOF packets to the client until we reached the end of 
the current statement.
This is a consolidation, to keep the functionality that is shared by all 
SQL statements in one place in the server.
Currently this functionality includes:
- close_thread_tables()
- log_slow_statement().

After this patch and the subsequent patch for Bug#12713, it shall also include:
- ha_autocommit_or_rollback()
- net_end_statement()
- query_cache_end_of_result().

In future it may also include:
- mysql_reset_thd_for_next_command().

sql/sql_update.cc

@@ -1696,7 +1696,11 @@ void multi_update::send_error(uint errcode,const char *err)
 {
   /* First send error what ever it is ... */
   my_error(errcode, MYF(0), err);
+}
 
+
+void multi_update::abort()
+{
   /* the error was handled or nothing deleted and no side effects return */
   if (error_handled ||
       !thd->transaction.stmt.modified_non_trans_table && !updated)