Failed to parse encrypted mp4 without "subsample_encryption_info" inside 'senc' box

[ktp420]

When trying to parse encrypted mp4 that only has 'iv' in 'sample_encryption_info' array inside 'SampleEncryptionBox', i.e. 'senc' type, then parser fails with following exception because the If condition is trying to look for 'flags' inside the 'sample_encryption_info' struct instade of the parrent struct 'SampleEncryptionBox'.

Traceback (most recent call last):
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 1089, in _parse
    return list(self.subcon._parse(stream, context, path) for i in range(count))
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 1089, in <genexpr>
    return list(self.subcon._parse(stream, context, path) for i in range(count))
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 859, in _parse
    subobj = sc._parse(stream, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 2696, in _parse
    return self.subcon._parse(stream, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 297, in _parse
    return self.subcon._parse(stream, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 1543, in _parse
    key = self.keyfunc(context) if callable(self.keyfunc) else self.keyfunc
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 1581, in <lambda>
    lambda ctx: bool(predicate(ctx)),
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/expr.py", line 145, in __call__
    context2 = self.__parent(context)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/expr.py", line 146, in __call__
    return context2[self.__name]
KeyError: 'flags'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 2696, in _parse
    return self.subcon._parse(stream, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 1093, in _parse
    raise RangeError("could not read prefix or enough elements, stream too short?")
construct.core.RangeError: could not read prefix or enough elements, stream too short?

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/ktp420/.venv/pywidevine/bin/mp4dump", line 8, in <module>
    sys.exit(dump())
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/pymp4/cli.py", line 26, in dump
    box = Box.parse_stream(fd)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 186, in parse_stream
    return self._parse(stream, context, "parsing")
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/pymp4/parser.py", line 46, in _parse
    obj = self.subcon._parse(stream2, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 855, in _parse
    subobj = list(sc._parse(stream, context, path).items())
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 297, in _parse
    return self.subcon._parse(stream, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 1544, in _parse
    obj = self.cases.get(key, self.default)._parse(stream, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 859, in _parse
    subobj = sc._parse(stream, context, path)
  File "/home/ktp420/.venv/pywidevine/lib/python3.10/site-packages/construct/core.py", line 2700, in _parse
    raise e.__class__("%s\n    %s" % (e, path))
construct.core.RangeError: could not read prefix or enough elements, stream too short?
    parsing -> sample_encryption_info