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FFI: Prevent sanitizer warning in carith_ptr().

Reported by Sergey Bronnikov.

(cherry picked from commit c3b379bf5)

The Undefined Behaviour Sanitizer [1] produce a warning about
signed integer overflow in the function carith_ptr(), when idx
equals INT_MIN in the expression idx = -idx. In practice,
negating -2147483648 wraps back to itself instead of producing the
correct positive value, leading to incorrect pointer arithmetic in
carith_ptr() for MM_sub (pointer subtraction). The patch fixes
that by converting idx to unsigned (uintptr_t), computes two's
complement negation via bitwise negation + 1 (safe from overflow
in unsigned arithmetic), and casts back to ptrdiff_t. This avoids
signed overflow entirely and correctly negates even the INT_MIN
edge case.

[1]: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

Sergey Bronnikov:
* added the description and the test for the problem

Part of tarantool/tarantool#12480

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