Return value of mysql_get_socket() indicating "no socket" not defined

mysql_get_socket() returns INVALID_SOCKET if there is no socket connected. However, INVALID_SOCKET is only defined internally within mysql, not in mysql.h. On Windows (and probably other BSD based systems), INVALID_SOCKET is defined by OS as -1, so is not a problem. CentOS does not have this, so there is no way to compare for no socket.