swscale/x86/input: prevent RGB32_TO_UV_FN from reading into the padding

This avoids reading uninitialized bytes and confusing valgrind

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/x86/input: prevent RGB32_TO_Y_FN from reading into the padding

This avoids reading uninitialized bytes and confusing valgrind

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