Fix a bug discovered by the synthetic video.

src/tscut/cutting.py

@@ -153,6 +153,8 @@ def cut_recording(media: PreparedMedia, options: ProcessingOptions,
         except InvalidMediaError:
             raise InvalidMediaError("Impossible to retrieve timestamp of final i-frame.")
 
+        assert head_iframe_ts <= tail_iframe_ts
+
         checks.append(pos+head_iframe_ts-ts1)
 
         subparts = []
@@ -199,6 +201,8 @@ def cut_recording(media: PreparedMedia, options: ProcessingOptions,
             if h264_head_ts is not None:
                 h264_ts.append(h264_head_ts)
 
+        if head_iframe_ts < tail_iframe_ts:
+
             # Creating MKV file that corresponds to current part between I-frames
             # Internal video with all streams (video, audio and subtitles)
             internal_mkv_name = f'part-{partnum:d}-internal.mkv'