mp3dec: Fix VBR bit rate parsing

When parsing the Xing/Info tag, don't set the bit rate if it's an Info tag.

When parsing the stream, don't override the bit rate if it's already set,
otherwise calculate the mean bit rate from parsed frames. This way, the bit
rate will be set correctly both for CBR and VBR streams.

CC:libav-stable@libav.org

Signed-off-by: Anton Khirnov <anton@khirnov.net>
