Make sfen reader only stop when it's destroyed. Now it is fully RAII.

src/learn/learn.cpp

@@ -502,8 +502,6 @@ namespace Learner
                 << "Error reading sfen_for_mse. Read " << sfen_for_mse.size()
                 << " out of " << sfen_for_mse_size << '\n';
 
-            sr.stop();
-
             return;
         }
 
@@ -541,8 +539,6 @@ namespace Learner
                 break;
         }
 
-        sr.stop();
-
         Eval::NNUE::finalize_net();
 
         save(true);