Fix ty_uniq case in maybe_auto_unbox

Closes #961
Issue #409

src/comp/middle/mut.rs

@@ -24,6 +24,7 @@ fn expr_root(tcx: ty::ctxt, ex: @expr, autoderef: bool) ->
               }
               ty::ty_uniq(mt) {
                 ds += [@{mut: mt.mut != imm, kind: unbox, outer_t: t}];
+                t = mt.ty;
               }
               ty::ty_res(_, inner, tps) {
                 ds += [@{mut: false, kind: unbox, outer_t: t}];

src/test/run-pass/unique-alt-discrim.rs

@@ -0,0 +1,8 @@
+// Issue #961
+
+fn altsimple() {
+    alt ~true {
+      _ { }
+    }
+}
+fn main() { }