This writes 4 bytes but in SSE4 mode only produces 2 bytes per vector. We
can avoid over-writing by using the appropriately sized register.
Reproducible by:
make libswscale/tests/swscale
libswscale/tests/swscale -dst monob -unscaled 1 -flags unstable -align_src 1 -align_dst 1
Signed-off-by: Niklas Haas <git@haasn.dev>