Fixes a floating-point exception further down.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>