Fix nonsense non-mod16 AMV flipping code.

It is obviously nonsense since it produces wrong results
or even crashes (crashes should be encode-only though).
Fixes trac issue #1092.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
