avcodec/dpx: Move need_align to act per line

Fixes out of array read
Fixes: 61cf123c081ee2bb774d307c75bdb99e/asan_heap-oob_1224f76_5546_bee833ffae73f752b489b9eeaac52db7.dpx

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
