CameraITS: Add a check for empty key frames list.

Bug: 233691518
Change-Id: Id383f3ec9782cc0d2c88e0f129eede8d45158b52
diff --git a/apps/CameraITS/utils/video_processing_utils.py b/apps/CameraITS/utils/video_processing_utils.py
index 187917a..210e699 100644
--- a/apps/CameraITS/utils/video_processing_utils.py
+++ b/apps/CameraITS/utils/video_processing_utils.py
@@ -92,6 +92,8 @@
   Returns:
     key_frame_file to be used for further processing.
   """
+  if not key_frame_files:
+    raise AssertionError('key_frame_files list is empty.')
   key_frame_files.sort()
   return key_frame_files[-1]