lavf: Initialize the stream info timestamps in avformat_new_stream

These are normally initialized to AV_NOPTS_VALUE at the start
of avformat_find_stream_info, but if a new stream is found while
this function is running (e.g. like in mpegts), the newly added
AVStreams didn't have these values properly initalized, leading
to avformat_find_stream_info terminating too soon (when the
first timestamps are far from 0).

Signed-off-by: Martin Storsjö <martin@martin.st>