evaluate_steady_state_file.m: Condition call to str2func on function being present

Workaround for Octave bug https://savannah.gnu.org/bugs/?52568

(cherry picked from commit 99e4cb6e)

matlab/evaluate_steady_state_file.m

@@ -70,7 +70,9 @@ else
     updated_params_flag = 0;
 end
 
+if M.set_auxiliary_variables %otherwise Octave may crash, see https://savannah.gnu.org/bugs/?52568
     h_set_auxiliary_variables = str2func([M.fname '_set_auxiliary_variables']);
+end
 if  isnan(updated_params_flag) || (updated_params_flag  && any(isnan(params(~isnan(params))-params1(~isnan(params))))) %checks if new NaNs were added
     info(1) = 24;
     info(2) = NaN;