Integer overflow in Unix.map_file

Summary

ANT-2026-6W9NPC2Q mmap dimension product integer overflow

## Fix

Use `caml_umul_overflow` instead of `*` when multiplying bigarray dimensions, which detects integer overflows and raise `Failure`. Negative positions are also rejected by raising `Invalid_argument`.

## Am I affected?

If either the dimensions or the file size in a call to `Unix.map_file` from your code is user controllable then you are potentially affected.

## Score notes

The vulnerability is in a library, and the CVSS score is calculated [under worst case assumptions](https://www.first.org/cvss/v4.0/user-guide#Assessing-Vulnerabilities-in-Software-Libraries-and-Similar). For an actual application the score will depend on how much an attacker can control the input to this function (if at all).

## Timeline

- 2026-08-24: vulnerability was found - 2026-08-27: forwarded report to security@ocaml.org - 2026-09-29: fix merged to trunk