bug fix: nonstationary vars and tex names

(cherry picked from commit aff56297)

preprocessor/ParsingDriver.cc

@@ -304,6 +304,9 @@ ParsingDriver::add_expression_variable(string *name)
 void
 ParsingDriver::declare_nonstationary_var(string *name, string *tex_name)
 {
+  if (tex_name != NULL)
+    declare_endogenous(new string(*name), new string(*tex_name));
+  else
     declare_endogenous(new string(*name), tex_name);
   declared_nonstationary_vars.push_back(mod_file->symbol_table.getID(*name));
   mod_file->nonstationary_variables = true;