Fix crash when stringifying pre 1970 dates on Windows

`gmtime*` on Windows fails on dates pre 1970, and because we didn't
check the return code, we would then pass invalid `tm` struct to
`strftime` causing it to assert.

Closes #2944

src/catch2/catch_tostring.hpp

@@ -634,7 +634,11 @@ struct ratio_string<std::milli> {
 
 #ifdef _MSC_VER
             std::tm timeInfo = {};
-            gmtime_s(&timeInfo, &converted);
+            const auto err = gmtime_s(&timeInfo, &converted);
+            if ( err ) {
+                return "gmtime from provided timepoint has failed. This "
+                       "happens e.g. with pre-1970 dates using Microsoft libc";
+            }
 #else
             std::tm* timeInfo = std::gmtime(&converted);
 #endif