[Nickle]union switch has unreachable default

[Bart Massey]

When I saw "union switch has unreachable default" I was very
impressed: quite cool to check!  Unfortunately, I saw it in
a context like

  typedef enum { V1, V2 } e;
  e v;
  union switch(v) {
   case V1:
       /* handle V1 */
       break;
   case V2:
       /* handle V2 */
       break;
   default:
       abort("somebody expanded the union datatype: fix me");
  }

As cool as it is, I'm afraid it has to go...

	Bart