reactor: fix crash during metrics gathering

The crash is caused by statistics request while scheduling_group is
removed. The tasks_pending metric gathering triggers
reactor::pending_task_count() call. This call causes null pointer
derefence as the task queue pointer has been reset in
reactor::destroy_scheduling_group()

src/core/reactor.cc

@@ -2545,7 +2545,9 @@ uint64_t
 reactor::pending_task_count() const {
  uint64_t ret = 0;
  for (auto&& tq : _task_queues) {
- ret += tq->_q.size();
+ if (tq) {
+ ret += tq->_q.size();
+ }
  }
  return ret;
 }