Prevent reached null conversion destinations from crashing sscanf_s and vsscanf_s. Return EOF before writes and retain earlier assignments. Add regression coverage through both guest ABI entry points.
AI-assisted: yes (OpenAI Codex). Windows WinLibs GCC 15.2 Release: production baseline crashes with the new regression; fixed shared libc passes guest_scan_count, guest_formatting, guest_wide_formatting and guest_strings (4/4). Conventions pass. Full all-target build was deliberately stopped after 83s with 3691 pending targets, not completed. No title or console run; constraint-handler invocation remains unimplemented as documented. No performance or coverage-percentage claim.
simonbronk