Bugfix for steady;

1.) Fixes cases where model was not truly linear, but Dynare did not catch mistake because fvec and jacob contained cancelling NaN
2.) Use correct error code 22 if NaN appear instead of generic 20
Merge pull request #418 from JohannesPfeifer/c82658a2a51ccbc02dfc658d99813950e76962e4

Fix for steady