curl: CVE-2026-86409: stale SigV4 headers sent unintended

Summary Setting CURLOPTAWSSIGV4 to NULL returns CURLEOK but does not disable SigV4 authentication. The handler in lib/setopt.c clears the parameter string STRINGAWSSIGV4 but never removes the CURLAUTHAWSSIGV4 bit from data-set.httpauth. A subsequent curleasyperform on the same handle still sends ...