Deal with desyncs in on_sync() call.

src/server_state.cpp

@@ -618,9 +618,15 @@ void wsrep::server_state::on_sync()
         };
     }
     else
+    {
+        // Calls to on_sync() in synced state are possible if
+        // server desyncs itself from the group. Provider does not
+        // inform about this through callbacks.
+        if (state_ != s_synced)
         {
             state(lock, s_synced);
         }
+    }
     init_synced_ = true;
 }