When a priority_queue is empty, the behavior of top() operator is

non-deterministic. Returns NULL when it's empty!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28560 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/ScheduleDAGList.cpp

@@ -356,6 +356,7 @@ public:
     }
     
     SUnit *pop() {
+      if (empty()) return NULL;
       SUnit *V = Queue.top();
       Queue.pop();
       return V;