Enforce std::time_base::mdy for "C" locale (#4437)

Co-authored-by: Stephan T. Lavavej <[email protected]>

Author: fsb4000

stl/src/xdateord.cpp

@@ -10,8 +10,13 @@
 _EXTERN_C_UNLESS_PURE
 
 _CRTIMP2_PURE int __CLRCALL_PURE_OR_CDECL _Getdateorder() noexcept { // return date order for current locale
+    const wchar_t* const locale_name = ___lc_locale_name_func()[LC_TIME];
+    if (locale_name == nullptr) { // Indicates "C" locale; GetLocaleInfoEx would see this as LOCALE_NAME_USER_DEFAULT
+        return std::time_base::mdy;
+    }
+
     wchar_t buf[2] = {0};
-    GetLocaleInfoEx(___lc_locale_name_func()[LC_TIME], LOCALE_ILDATE, buf, static_cast<int>(std::size(buf)));
+    GetLocaleInfoEx(locale_name, LOCALE_ILDATE, buf, static_cast<int>(std::size(buf)));
 
     switch (buf[0]) {
     case L'0':