Port r110 from branches/5.0:

Do not log empty binlog file name at transaction commit.

trx_commit_off_kernel(): Do not write empty trx->mysql_log_file_name.

trx/trx0trx.c

@@ -794,7 +794,8 @@ trx_commit_off_kernel(
 		in trx sys header if MySQL binlogging is on or the database
 		server is a MySQL replication slave */
 
-		if (trx->mysql_log_file_name) {
+		if (trx->mysql_log_file_name
+				&& trx->mysql_log_file_name[0] != '\0') {
 			trx_sys_update_mysql_binlog_offset(
 					trx->mysql_log_file_name,
 					trx->mysql_log_offset,