fix array index overflow in set_at_pointer

[Ramya-9353]

Repro: call set_at_pointer on a non-empty array with an index token equal to std::size_t(-1), e.g. "/18446744073709551615" on a 64-bit build, and a max_created_elements large enough to pass the element-count guard.

Cause: the guard bounds n (the number of elements to create) but not index + 1. arr.size() + n + 1 is index + 1, which wraps to 0, so resize takes the shrink branch (skipping the array_too_large check) and the lambda returns arr.data() + index, a pointer one element before the buffer. The following *result = ref.make_value(...) then writes through it (ASAN: heap-buffer-overflow in value::operator= via pointer.ipp:490; UBSan: unsigned pointer overflow at pointer.ipp:462).

Fix: reject index >= array::max_size() before the resize and fail with error::array_too_large, next to the existing max_created_elements check. Regression test drives index std::size_t(-1) on a non-empty array and checks the array is untouched and error::array_too_large is returned.

[cppalliance-bot]

An automated preview of the documentation is available at https://1192.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-08-27 09:39:05 UTC

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GCOVR code coverage report https://1192.json.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://1192.json.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://1192.json.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-08-27 09:49:19 UTC

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Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1192/pullrequest.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.72%. Comparing base (e0e74f6) to head (4be012d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1192   +/-   ##
========================================
  Coverage    93.72%   93.72%           
========================================
  Files           85       85           
  Lines         8981     8983    +2     
========================================
+ Hits          8417     8419    +2     
  Misses         564      564           

Files with missing lines	Coverage Δ
include/boost/json/impl/pointer.ipp	100.00% <100.00%> (ø)

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Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1192/pullrequest.html