Fix a compiler warning

Clang things that timings can be NULL here,
and I think it might be right.

gdk/gdkframeclockidle.c

@@ -622,7 +622,10 @@ gdk_frame_clock_paint_idle (void *data)
             }
 #ifdef G_ENABLE_DEBUG
             if (GDK_DEBUG_CHECK (FRAMES))
-              timings->frame_end_time = g_get_monotonic_time ();
+              {
+                if (timings)
+                  timings->frame_end_time = g_get_monotonic_time ();
+              }
 #endif /* G_ENABLE_DEBUG */
           G_GNUC_FALLTHROUGH;