swresample/rematrix_template: Fix integer overflow in mix6to2

Fixes: CID1030351
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swresample/rematrix_template: Fix integer overflow in mix8to2

Fixes: CID1030352
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>