Latent bug in iter_structural_ty: handle _match::Single on zero-variant enum.

(This may not be the *best* fix, compared to e.g. returning
`_match::NoBranch` from `trans_switch` on a zero-variant enum. But it
is one of the *simplest* fixes available.)

Author: pnkfelix

src/librustc_trans/trans/adt.rs

@@ -769,6 +769,7 @@ pub fn trans_switch<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
             (_match::Switch, Some(trans_get_discr(bcx, r, scrutinee, None)))
         }
         Univariant(..) => {
+            // N.B.: Univariant means <= 1 enum variants (*not* == 1 variants).
             (_match::Single, None)
         }
     }

src/librustc_trans/trans/base.rs

@@ -471,8 +471,11 @@ pub fn iter_structural_ty<'blk, 'tcx, F>(cx: Block<'blk, 'tcx>,
 
           match adt::trans_switch(cx, &*repr, av) {
               (_match::Single, None) => {
-                  cx = iter_variant(cx, &*repr, av, &*(*variants)[0],
-                                    substs, &mut f);
+                  if n_variants != 0 {
+                      assert!(n_variants == 1);
+                      cx = iter_variant(cx, &*repr, av, &*(*variants)[0],
+                                        substs, &mut f);
+                  }
               }
               (_match::Switch, Some(lldiscrim_a)) => {
                   cx = f(cx, lldiscrim_a, cx.tcx().types.isize);