fixed empty trace

src/aion/Tracer.cc

@@ -106,6 +106,9 @@ uint32_t* detail::alloc_chunk()
 
 void visit_thread(visitor& v)
 {
+    if (profile_local_chunks.empty())
+        return; // DEBUG
+
     v.on_thread(std::this_thread::get_id());
 
     auto curr_chunk = 0u;