do not return a file if we do not find a start file

prolefeeder.py

@@ -78,6 +78,8 @@ def prepare_download(form):
     for i, source in enumerate(sources):
         if source['start_time'] <= form.start_time.data < source['start_time'] + source['length']:
             start_index = i
+    if start_index is None:
+        return None
     sources = sources[start_index:]
 
     # Seek into the first file