Fix feedback strategy pct (#830)

Author: ChristineZh0u

Src/PChecker/CheckerCore/SystematicTesting/Strategies/Feedback/PCTScheduler.cs

@@ -209,6 +209,10 @@ private void PreparePriorityChangePoints(int step)
                 int index = _randomValueGenerator.Next(listOfInts.Count);
                 PriorityChangePoints.Add(listOfInts[index]);
                 listOfInts.RemoveAt(index);
+                if (listOfInts.Count == 0)
+                {
+                    break;
+                }
             }
         }