Fix bug in dyn_ramsey_static.m with steady state file that resulted in incorrect steady state when running twice

The x returned by a steady state file includes Lagrange multipliers. When starting with 0 multipliers, the steady state of the multipliers is computed correctly. But when running it a second time like in a subsequent call to stoch_simul, the algorithm thinks that elements of xx are 0 which they aren't. As a result, it finds minus the multiplier as the solution, resulting in an output of 0 for all multipliers.