check gmtime_r return in httpGetDateString

[tanjiroK-coder]

httpGetDateString takes a time_t straight from the caller and feeds it to gmtime_r without checking the result. When the value falls outside the range gmtime_r can represent it returns NULL and leaves tdate untouched, and the code then uses tdate.tm_wday and tdate.tm_mon as indices into the 7- and 12-element http_days/http_months tables and hands the loaded pointer to %s, so an uninitialised tm_mon becomes an out-of-bounds read and a wild pointer dereference. The reachable path is cups-oauth, which passes (time_t)cupsJSONGetNumber(value) from a JWT exp/iat/nbf claim, so an id_token carrying exp: 9e18 is enough. I hit it reading the code after bb7b1e5 and confirmed gmtime_r returns NULL there; a build against the sanitiser faults with a SEGV inside httpGetDateString on that input. This is the same fix bb7b1e5 applied to the sibling ippTimeToDate, so I mirrored it here: zero the struct and set tm_mday to 1 when gmtime_r fails.

Assisted-by: Claude Code:Opus-4.8 [Claude Code]

[michaelrsweet]

Thanks!

[master 6b47442] Fix httpGetDateString with bad date/time values (Issue #167)