This function overreads the input plane for odd dimensions, because the
chroma plane is always rounded up, which means (xy << subsample) + 1 exceeds
the actual alpha plane size.
To verify:
valgrind ffmpeg -pix_fmt yuva420p -f lavfi -i color -vf \
"scale=1x1,format=yuva420p,scale=alphablend=uniform_color,format=yuv420p \
-vframes 1 -f null -
Fixes: https://trac.ffmpeg.org/ticket/11692
b7946098b1