BitStreamFilter initialize failed

[cmpute]

To reproduce: python -c "import av; print(av.bitstream.BitStreamFilterContext('h264_mp4toannexb'))"

Error:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "av/bitstream.pyx", line 33, in av.bitstream.BitStreamFilterContext.__cinit__
  File "av/error.pyx", line 424, in av.error.err_check
av.error.ValueError: [Errno 22] Invalid argument

After some quick investigation, it might be related to av_bsf_init(self.ptr) was called without av_bsf_alloc being called. See https://github.com/PyAV-Org/PyAV/blob/main/av/bitstream.pyx#L32 and https://ffmpeg.org/doxygen/trunk/group__lavc__bsf.html#ga242529d54013acf87e94273d298a5ff2

[cmpute]

After enabling logging, here's the error log from libav:

Codec 'unknown' (0) is not supported by the bitstream filter 'h264_mp4toannexb'. Supported codecs are:
h264 (27)

Traceback (most recent call last):
  File "/home/jacobz/workspace/solarix-depot/test_pyav2.py", line 11, in <module>
    ctx = BitStreamFilterContext("h264_mp4toannexb")
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "av/bitstream.pyx", line 33, in av.bitstream.BitStreamFilterContext.__cinit__
  File "av/error.pyx", line 424, in av.error.err_check
av.error.ValueError: [Errno 22] Invalid argument; last error log: [h264_mp4toannexb]