avformat: fix overflows during bit rate calculation

The bit_rate field has type int64_t since commit
7404f3bdb90e6a5dcb59bc0a091e2c5c038e557d.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
