avformat/utils: Fix bitrate overflow check

The check added in df33a58e5311ee9a64a573889b883a80e981af7b does not work
at all, rather it broke the summing of bitrates completely.
The comparission was wrong way around.
This commit replaces it by a simpler and hopefully clearer check

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
