Fix message in dynare_estimation_init.m

If some parameters are NaN after computing the steady state, the message erroneously said that parameters had been changed. In a check for equality, NaN is not equal to NaN. Thus, NaNs are now taken care of separately.
Merge pull request #765 from JohannesPfeifer/analytic_derivation_message

Fix message in dynare_estimation_init.m