Fix non-atomic refcount decrement

[aviggiano]

Summary

• compare the decremented reference count, rather than its pointer, in the non-atomic fallback
• add a dedicated regression test that forces CROARING_ATOMIC_IMPL_NONE

Root cause and impact

croaring_refcount_dec() decremented *val but returned val == 0. Since callers pass a valid pointer, the function could never report that the final reference had been released. This prevents shared containers from being freed and makes extraction take an unnecessary copy path when the non-atomic fallback is selected.

Fixes #858.

This was found while reviewing the vendored CRoaring integration in redis/redis#15331.

Validation

• confirmed the new regression test fails on unmodified master
• cmake -S . -B build-release -DCMAKE_BUILD_TYPE=Release -DENABLE_ROARING_TESTS=ON
• cmake --build build-release -j2
• ctest --test-dir build-release --output-on-failure — 27/27 passed
• forced-fallback test compiled as C++ with Clang 18 and passed
• git diff --check

The Docker-based formatting helper was unavailable locally; formatting remains covered by CI.

Prepared with Codex assistance under contributor direction; the bug was reproduced and the complete test suite above was run before opening this draft.

[lemire]

Seems like an obvious defect to me. Merging.