bug#4997 - --log-error sometimes generates incorrect default name.

Now replacing domain name by 'err'. If host name doesn't have a
domain name, '.err' is appended as before.

sql/mysqld.cc

@@ -2535,7 +2535,8 @@ server.");
   if (opt_error_log)
   {
     if (!log_error_file_ptr[0])
-      fn_format(log_error_file, glob_hostname, mysql_data_home, ".err", 0);
+      fn_format(log_error_file, glob_hostname, mysql_data_home, ".err",
+                MY_REPLACE_EXT); /* replace '.<domain>' by '.err', bug#4997 */
     else
       fn_format(log_error_file, log_error_file_ptr, mysql_data_home, ".err",
 		MY_UNPACK_FILENAME | MY_SAFE_PATH);