avfilter/vf_spp: Fix overflow in 8bit store slice

Fixes regression with
ffplay -f lavfi -i testsrc=640x480  -vf format=gray,boxblur=20:10,geq="'mod(lum(X,Y),16)*15'",boxblur=10,geq="'abs(mod(lum(X,Y),15)-7)*32'",spp=4:40

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