Actually, we can do this in constant time. xofs always
contains same or increasing offset values. We can instead
find the most extreme value used and never attempt to load it.

Similarly, we can note for all dx >= 0 and dx < (dwidth - cn)
where xofs[dx] + cn < xofs[dwidth-cn] implies dx < (dwidth - cn).

Thus, we can use this to control our loop termination optimally.

This fixes #16137 with little or no performance impact. I have
also added a debug check as a sanity check.