fixed prepearbucket function which can lead to non-optimal plans for …

…bidirectional search in rare cases

src/search/symbolic/searches/uniform_cost_search.cc

@@ -155,7 +155,10 @@ void UniformCostSearch::stepImage(int maxTime, int maxNodes) {
             }
         }
     }
-    prepareBucket();
+
+    while (!frontier.bucketReady() && !open_list.empty()) {
+        prepareBucket();
+    }
 
     engine->setLowerBound(getG() + mgr->getAbsoluteMinTransitionCost());
     step_estimation.set_data(step_timer(), stepNodes, !res_expansion.ok);