MORTEVIELLE: Fix out-of-bounds read

This should be sufficient. At this point, cy is at most 23, and if
cx is 41 it's either 23 or less than 20. That means ix is either
46 or less than 43. If it's less than 43, there is no out of bounds
access, and now we look at the special case first.

This was a Coverity issue, but for some reason I don't see any CID
for it.