enb,stack_lte: protect access to sync queue

the sync queue is only created during init so access needs to be protected

srsenb/src/stack/enb_stack_lte.cc

@@ -203,8 +203,10 @@ int enb_stack_lte::init(const stack_args_t&      args_,
 
 void enb_stack_lte::tti_clock()
 {
+  if (started.load(std::memory_order_relaxed)) {
     sync_task_queue.push([this]() { tti_clock_impl(); });
   }
+}
 
 void enb_stack_lte::tti_clock_impl()
 {