dvdplayer: do not overwrite stream samplerate with zero

If the codec reports a sample rate of 0, do not use it to overwrite
stream sample rate from demuxer.

Found by elupus.
dvdplayer: add proper default implementation for GetEncodedSampleRate()

For non-passthrough codecs the content sample rate matches output sample
rate, so have GetEncodedSampleRate() return GetSampleRate().

Also drop the now unneeded specific implementation in
DVDAudioCodecFFmpeg.
Merge pull request #4672 from anssih/fix/dvdplayer-samplerate

dvdplayer sample rate fixes