Fix some uninitialized variables with gensfen

fixes valgrind errors as seen with:

```
setoption name Use NNUE value true
isready
gensfen depth 6 loop 10 use_draw_in_training_data_generation 1 eval_limit 32000 output_file_name training_data/training_data.bin use_raw_nnue_eval 0
quit
```

the latter script now runs without valgrind errors on linux

src/learn/gensfen.cpp

@@ -1159,6 +1159,8 @@ namespace Learner
         // Show if the training data generator uses NNUE.
         Eval::verify_NNUE();
 
+        Threads.main()->ponder = false;
+
         // Create and execute threads as many as Options["Threads"].
         {
             SfenWriter sfen_writer(output_file_name, thread_num);