do not join() if on sync thread

Adapter.cc

@@ -51,11 +51,13 @@ namespace pEp {
                 return 1;
             }
             if (ev == nullptr) {
+                if (!on_sync_thread()) {
                     if (_sync_thread) {
                         _sync_thread->join();
                         delete _sync_thread;
                         _sync_thread = nullptr;
                     }
+                }
                 q.clear();
             }
             return 0;