Added a test case for Silicon

src/test/resources/all/issues/silicon/0053.sil

@@ -0,0 +1,20 @@
+var next: Ref
+var val: Int
+
+predicate valid(this: Ref) {
+  acc(this.next) && acc(this.val) &&
+  (this.next != null ==> acc(valid(this.next)))
+}
+
+method traverse(list: Ref)
+  requires acc(valid(list))
+{
+  var tmp: Ref := list
+
+  while (tmp != null)
+    invariant (tmp != null) ==> acc(valid(tmp))
+  {
+    unfold acc(valid(tmp))
+    tmp := tmp.next
+  }
+}