demux_lavf: workaround for broken libavformat subtitle seeking

The really funny thing about this commit is that this code is added on
top of another work around. Basically, subtitle seeking in libavformat
is completely broken. To make it useful, we have to add yet another
workaround.

The basic problem is that libavformat's subtitle seeking code always
uses the stream time base, instead of AV_TIME_BASE if stream index -1 is
passed to the avformat_seek_file() function.

Fixes github issue #216. Hopefully this will be fixed in ffmpeg too at
some point.