Avoid crash when an unknown object is declared as a type.

From-SVN: r168188

gcc/go/gofrontend/expressions.cc

@@ -1248,7 +1248,12 @@ Unknown_expression::do_lower(Gogo*, Named_object*, int)
 {
   source_location location = this->location();
   Named_object* no = this->named_object_;
-  Named_object* real = no->unknown_value()->real_named_object();
+  Named_object* real;
+  if (!no->is_unknown())
+    real = no;
+  else
+    {
+      real = no->unknown_value()->real_named_object();
       if (real == NULL)
 	{
 	  if (this->is_composite_literal_key_)
@@ -1257,6 +1262,7 @@ Unknown_expression::do_lower(Gogo*, Named_object*, int)
 		   this->named_object_->message_name().c_str());
 	  return Expression::make_error(location);
 	}
+    }
   switch (real->classification())
     {
     case Named_object::NAMED_OBJECT_CONST: