Cleanup execute_ddl_log_recovery() to not generate an error if

there is nothing to recover.

Discovered while working on Bug#12713


sql/sql_table.cc:
  Use MYF(0) in my_open() in read_ddl_log_header() called from
  execute_ddl_log_recovery() called during the server start up to not
  generate an error if no ddl log exists. This is not an erroneous situation,
  in fact it's the case in any server statrtup. The error was lost
  anyway, since it was pushed into the stack of the artificial thd.

sql/sql_table.cc

@@ -434,7 +434,7 @@ static uint read_ddl_log_header()
 
   create_ddl_log_file_name(file_name);
   if ((global_ddl_log.file_id= my_open(file_name,
-                                        O_RDWR | O_BINARY, MYF(MY_WME))) >= 0)
+                                        O_RDWR | O_BINARY, MYF(0))) >= 0)
   {
     if (read_ddl_log_file_entry(0UL))
     {