Sometimes the input buffers get directly used as raw images and
SIMD optimized video/image filters can sometimes read more than 16 bytes
over the end.
a specific example is the AVX 24bpp to yuv code

This also fixes fate-vsynth3-rgb
Reviewed-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>