Eol_compat: fix crash triggered by ocamlgrammarfuzzer

[let-def]

Eol_compat assumes that the excluded locations are non‑overlapping and strictly increasing. This assumption does not hold in all cases, as illustrated by the following sentence:

object method ! x1 : type x2 . x3 * {%ext|s|} = X4 end

The proposed change does not introduce a regression: it only triggers for situations that were not previously covered. However, I do not fully understand Eol_compat, so I cannot guarantee that it behaves as intended on sentences that were failing before.

This fix is a prerequisite for integrating the fuzzer, because the current issue makes ocamlformat crash and prevents further analysis. (Exceptions are usually caught by the ocamlformat driver and turned into an error message, not this one.)