Ensure FFmpeg doesn't corrupt an early keyframe

When the analyze func runs, it seems to check between 2-4% (or so?) of the stream, which for longer videos especially we don't have yet.  Returning a read failure (tried -1 or 0) here seems to truncate that keyframe sometimes.

FFmpeg asks for the total size when this happens, which we were ignoring (docs say it's okay to do that, though.)  Handling it fixes this, as FFmpeg apparently realizes we ran out of data.

I'm a bit busy atm, but Patapon 3 and Wipeout Pure still work with this, and @thedax tested a bunch of games for me and they were all happy (thanks.)

-[Unknown]