dev/cppcheck: add minimal reproducer for memleakOnRealloc false positive

[Copilot]

cppcheck incorrectly reports [memleakOnRealloc] in src/quic_rx.c:819 (qc_try_store_new_token) because it doesn't track that a local copy of a struct-held pointer preserves the original allocation through the failure path.

Changes

• dev/cppcheck/cppcheck_test.c — self-contained C file (no haproxy headers) that reproduces the false positive with stripped-down types mirroring the original pattern:

local_ptr = buf_ptr(*b);          // local copy of b->ptr
if (len > buf_len(*b)) {
    local_ptr = realloc(local_ptr, len);  // cppcheck fires here
    if (local_ptr)
        b->ptr = local_ptr;
    else {
        memset(buf_ptr(*b), 0, buf_len(*b));
        buf_free(b);              // b->ptr still valid, correctly freed
    }
}

cppcheck sees local_ptr nulled on realloc failure and assumes the original allocation is lost, not realising b->ptr still holds it and the else branch frees it. The file is ready to submit as a cppcheck upstream bug report.