snow: Fix off by 1 error in reference picture management

Fixes out of array accesses
No release is affected by this bug

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
evrcdec: fix sign error

The specification wants round(abs(x))) * sign(x) which is
equivakent to round(x)

Fixes out of array access

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>