Fix top-pointer help (potentially not lock free without the fix).

lib/pls/include/pls/internal/scheduling/lock_free/traded_cas_field.h

@@ -46,8 +46,6 @@ struct traded_cas_field {
     return stamp_;
   }
   [[nodiscard]] unsigned long get_trade_request_thread_id() const {
-    PLS_ASSERT(is_filled_with_trade_request() || is_filled_with_object(),
-               "Must only read out the tag when the traded field contains one.");
     return trade_request_thread_id_ - 1;
   }
 
@@ -62,6 +60,9 @@ struct traded_cas_field {
   [[nodiscard]] bool is_filled_with_object() const {
     return non_empty_flag_ && traded_task_id_;
   }
+  [[nodiscard]] bool object_was_traded_in() const {
+    return traded_task_id_ != 0;
+  }
   [[nodiscard]] bool is_empty() const {
     return !non_empty_flag_;
   }

lib/pls/src/internal/scheduling/lock_free/external_trading_deque.cpp

@@ -135,7 +135,8 @@ task *external_trading_deque::pop_top(task *offered_task, peek_result peek_resul
                                  std::memory_order_acq_rel);
     return result;
   } else {
-    if (expected_sync_cas_field.is_filled_with_object() && expected_sync_cas_field.get_stamp() == expected_top.stamp_
+    if (expected_sync_cas_field.object_was_traded_in()
+        && expected_sync_cas_field.get_stamp() == expected_top.stamp_
         && expected_sync_cas_field.get_trade_request_thread_id() == thread_id_) {
       top_.compare_exchange_strong(expected_top,
                                    {expected_top.stamp_ + 1, expected_top.value_ + 1},