Wrong frames count with PyFfmpegDecoder #536

import PyNvCodec as nvc
import numpy as np

gt_num_frames = 299
decoders = [nvc.PyNvDecoder("video.mkv", 0), nvc.PyFfmpegDecoder("video.mkv", {})]
frame = np.ndarray(shape=(0), dtype=np.uint8)

def decode_all_frames(dec, num_frames):
    my_frames = 0
    while(True):
        if not dec.DecodeSingleFrame(frame):
            break
        my_frames += 1
    assert num_frames == my_frames, f"{dec.__class__.__name__} got {my_frames} should be {num_frames}"

for dec in decoders:
    decode_all_frames(dec, gt_num_frames)