Fix the 84 NO / int overflow failure on Corax's test rom

[Paul]

This test appears to check whether integers wrap correctly:

#test 8xy4
y := 26
drawop im8 im4
i := imagefalse
v6 := 140
v7 := 140
v7 += v6
if v7 == 24 then i := imageok
sprite x2 y 4

It's probably broken clamping. It may also require altering tests.