Fix minor Kaldi bug in AudioStore.save_all() method

This stops save_all() raising an error if deque is None.

Author: drmfinlay

dragonfly/engines/backend_kaldi/audio.py

@@ -298,6 +298,9 @@ def save(self, index):
                 ]) + '\n')
 
     def save_all(self):
+        if self.deque is None:
+            return
+
         for i in reversed(range(len(self.deque))):
             self.save(i)