DVDAudioCodecFFmpeg: Convert unknown audio formats to float.

As of FFmpeg-1.1, some decoders started to output planar audio.
This fixes track ticket 13944.

This will be needed when we'll update the internal FFmpeg to a newer release.
It's not my call but it'd be cool if this were backported to Frodo as it fixes a crash.

TODO: I fear passthrough is affected too, will have a look, but this one was easy enough.