Fixed variable lifetime in loops

src/build/ast/Parse.go

@@ -30,6 +30,10 @@ func toASTNode(tokens token.List) (Node, error) {
 		word := tokens[0].Text()
 
 		if word == keyword.Return {
+			if len(tokens) == 1 {
+				return &Return{}, nil
+			}
+
 			value := expression.Parse(tokens[1:])
 			return &Return{Value: value}, nil
 		}