Add flag if lyapunov_symm fails

In models like the one at http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=6291 lyapunov_symm fails to solve the Lyapunov equation and outputs NaN. This error is not flagged and no exception thrown out, leading to subsequent crashes. We should add a check whether the solver was successful and only continue if so.