gdatetime: Fix error handling in g_date_time_new_ordinal()

It was possible to pass in (for example) an invalid hour to
g_date_time_new_ordinal(), which would be passed on to
g_date_time_new(), which would (correctly) return `NULL` — but then
g_date_time_new_ordinal() would try to dereference that.

Includes some test cases.

oss-fuzz#16103
oss-fuzz#17183

Signed-off-by: Philip Withnall <withnall@endlessm.com>

glib/gdatetime.c

@@ -1220,6 +1220,8 @@ g_date_time_new_ordinal (GTimeZone *tz, gint year, gint ordinal_day, gint hour,
     return NULL;
 
   dt = g_date_time_new (tz, year, 1, 1, hour, minute, seconds);
+  if (dt == NULL)
+    return NULL;
   dt->days += ordinal_day - 1;
 
   return dt;