Don't completely fill the psmf ringbuffer at once.

It takes more time (slowing the start of the video), and gets us closer to
overflowing buffers and stuff.
Check the audio buffer size just in case.

This way we definitely won't overfill it.
Merge pull request #5002 from unknownbrackets/mpeg-minor

Prevent psmfplayer from loading too many mpeg frames at once