If nb_samples is zero, the bit_rate calculation results in a division by
zero.

Since ff_get_buffer fails if frame->nb_samples is zero, this can be
fixed by moving the bit_rate calculation after that function call.

That also makes it possible to reuse the already calculated
frame->nb_samples value.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>