Decoding frames before the first frame starts returns all the frames

[NicolasHug]

(This bug is related but orthogonal to #553, which is now fixed)

If the first frame of a stream is at pts=S and we ask for samples with stop_pts <= S, the decoder actually returns all the samples in the stream. For example in our mp3 test asset which starts at S=0.13:

frames, *_ = get_frames_by_pts_in_range_audio(decoder, start_seconds=0, stop_seconds=0.05)
all_frames, *_ = get_frames_by_pts_in_range_audio(decoder, start_seconds=0, stop_seconds=None)

frames and all_frames are equal, while we would expect frames to be empty.