Convert object creation statement into assignment

As the creation of new objects is free of side-effects in MiniJava, it
is not valid to create new objects without using them as part of an
assignment or a method invocation.

exec/new_object.java

 class new_object {
     public static void main(String[] args){
-        new new_object();
+        new_object obj = new new_object();
     }
-}
\ No newline at end of file
+}