Fixed Dataset.subset when buffer_size is None

Author: varisd

neuralmonkey/dataset.py

@@ -616,5 +616,6 @@ def subset(self, start: int, length: int) -> "Dataset":
             iterators=slices,
             batching=self.batching,
             outputs=outputs,
-            buffer_size=buffer_size,
+            buffer_size=((self.buffer_min_size, self.buffer_size)
+                         if self.lazy else None),
             shuffled=self.shuffled)