Allow messages to nil.

That is, [nil message];. I just found out this is "perfectly legal"
(http://www.otierney.net/objective-c.html).

tools/qfcc/source/expr.c

@@ -2818,6 +2818,8 @@ message_expr (expr_t *receiver, keywordarg_t *message)
 				class_msg = 1;
 				receiver = new_symbol_expr (class_pointer_symbol (class));
 			}
+		} else if (receiver->type == ex_nil) {
+			convert_nil (receiver, &type_id);
 		}
 		rec_type = get_type (receiver);