usm: Assert duration_time to be present (#4)

Author: mos9527

PyCriCodecsEx/usm.py

@@ -262,8 +262,9 @@ def frames(self):
         for i, frame in enumerate(self.packets):            
             frame_size = offsets[i + 1] - offsets[i]
             self.file.seek(offsets[i])
-            raw_frame = self.file.read(frame_size)
-            yield raw_frame, frame, frame.get("flags", [None])[0] == "K", float(frame.get("duration_time", 0))
+            raw_frame = self.file.read(frame_size)            
+            assert 'duration_time' in frame, "Frame duration missing. Consider re-encoding?"
+            yield raw_frame, frame, frame["flags"][0] == "K", float(frame["duration_time"])
 
     def generate_SFV(self, builder: "USMBuilder"):        
         v_framerate = int(self.framerate)