gc.hpp: Explicitly load tail->_next

rt/include/typon/fundamental/gc.hpp

@@ -111,11 +111,16 @@ namespace typon::fdt::lock_free
     {
       while (auto tail = _tail.load())
       {
-        if (tail->_stamp >= oldest || !tail->_next)
+        if (tail->_stamp >= oldest)
         {
           break;
         }
-        if (_tail.compare_exchange_strong(tail, tail->_next))
+        auto next = tail->_next.load();
+        if (!next)
+        {
+          break;
+        }
+        if (_tail.compare_exchange_strong(tail, next))
         {
           delete tail;
         }