NULL check on default keymap

Ensure that Quartz's default keymap is not accessed before it is
created.

gdk/quartz/gdkkeys-quartz.c

@@ -492,6 +492,7 @@ update_keymap (void)
         p[0] = known_numeric_keys[i].keypad_keyval;
     }
 
+  if (default_keymap != NULL)
     g_signal_emit_by_name (default_keymap, "keys-changed");
 }