Hi!

When creating hls playlist from video, duration of last segment in variant playlist calculated incorrectly. When last segment is one sample only, this cause to create playlists with zero duration chunks `#EXTINF:0.000000,` which cause `-16240` error on ios 14.2+ devices while trying download content.


Here command to reproduce issue:
```(sh)
wget https://storage.yandexcloud.net/github-public/video_sdr_avc_1280x534_2400_4000_720p.mp4
mp42hls --hls-version 7 --segment-duration 4 --verbose --segment-filename-template %d.ts --segment-url-template %d.ts --index-filename video_sdr_avc_720p.m3u8 --iframe-index-filename video_sdr_avc_720-iframes.m3u8 video_sdr_avc_1280x534_2400_4000_720p.mp4
```

Result playlist contains zero duration segment:
```(sh)
tail video_sdr_avc_720p.m3u8
57.ts
#EXTINF:4.000000,
58.ts
#EXTINF:4.000000,
59.ts
#EXTINF:4.000000,
60.ts
#EXTINF:0.000000,
61.ts
#EXT-X-ENDLIST
```
Incorrect last segment duration calculation in mp42hls
This is a GitHub Issue
repo:Bento4
owner:axiomatic-systems
Title : Incorrect last segment duration calculation in mp42hls
Issue date:

--- start body ---
Hi!

When creating hls playlist from video, duration of last segment in variant playlist calculated incorrectly. When last segment is one sample only, this cause to create playlists with zero duration chunks `#EXTINF:0.000000,` which cause `-16240` error on ios 14.2+ devices while trying download content.


Here command to reproduce issue:
```(sh)
wget https://storage.yandexcloud.net/github-public/video_sdr_avc_1280x534_2400_4000_720p.mp4
mp42hls --hls-version 7 --segment-duration 4 --verbose --segment-filename-template %d.ts --segment-url-template %d.ts --index-filename video_sdr_avc_720p.m3u8 --iframe-index-filename video_sdr_avc_720-iframes.m3u8 video_sdr_avc_1280x534_2400_4000_720p.mp4
```

Result playlist contains zero duration segment:
```(sh)
tail video_sdr_avc_720p.m3u8
57.ts
#EXTINF:4.000000,
58.ts
#EXTINF:4.000000,
59.ts
#EXTINF:4.000000,
60.ts
#EXTINF:0.000000,
61.ts
#EXT-X-ENDLIST
```
--- end body ---
