no dash output using develop branch

[flnewsoft]

env: branch: develop
git : commit 6bbd461
how to reproduce:

1. objs/srs -c conf/dash.conf
2. send rtmp stream to 1935 using ffmpeg
3. cd objs/nginx/html, no mpd file exist.
4. cd objs/nginx/html/live/stream, only audio-init.mp4 video-init.mp4 found.

[suzp1984]

Because you are publish a only video or only audio media resource.
Try another stream with both audio and video track, then after 3 fragment period, around 30 seconds, the mpd file will appear.

But it's a bug, let dash support single audio or single video stream.