message: add an assertion
diff: @@ -238,6 +238,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  match this.machine.threads.sync.rwlocks[id].readers.entry(reader) {
  Entry::Occupied(mut entry) => {
  let count = entry.get_mut();
  + assert!(*count > 0, "rwlock locked with count == 0");
  *count -= 1;
  if *count == 0 {
  entry.remove();
  