Fix tests that fail when the MySQL's socket file is not /tmp/mysql.sock

Fix tests that fail when the MySQL's socket file is not /tmp/mysql.sock
as it is in when compiled from source and the default for mysqlnd.
SuSE for example uses /var/run/mysql/mysql.sock . Also, sql.safe_mode
(ext/mysql and ingres) needs the socket.
Fix possible crashes in mysqlnd. When packets are shorter, functions should
return error.