fixed: cur_dts returned by ffmpeg is invalid most of the time so relay o...

...n packet dts instead. Fixes ticket #14535
I think it is wrong iterating over all streams. Once ffmpeg has added a stream to the array it stays there and never gets removed. there may be inactive streams in the array. 
for seeking we instruct ffmpeg to use default stream. imo we should read cur_dts only from default stream. can you try this? use av_find_default_stream_index to get index of default stream