MythMusic: fix playback of ogg radio streams

Peek the probe data from the stream buffer rather than read the data otherwise
ffmpeg fails to open the stream because the magic it is looking for has already
been read from the stream.

Also increase the probe buffer to 16K this seems to be required for some ogg
streams to be probed properly.

(cherry picked from commit 6b09d7c629e4efac81276eee9f4b7cbe3bc661ef)