Fix difficulty check assert in work_pool (#1958)

nano/lib/work.cpp

@@ -114,7 +114,7 @@ void nano::work_pool::loop (uint64_t thread)
 			if (ticket == ticket_l)
 			{
 				// If the ticket matches what we started with, we're the ones that found the solution
-				assert (output >= network_constants.publish_threshold);
+				assert (output >= current_l.difficulty);
 				assert (work_value (current_l.item, work) == output);
 				// Signal other threads to stop their work next time they check ticket
 				++ticket;