reprotect _mixer_popup_key_down_cb

/me still asleep, it's better like this

src/modules/mixer/e_mod_main.c

@@ -467,6 +467,7 @@ _mixer_popup_key_down_cb(void *data, Ecore_Event_Key *ev)
    E_Mixer_Instance *inst = data;
    const char *keysym;
 
+   if (!inst) return ECORE_CALLBACK_PASS_ON;
    keysym = ev->key;
    if (strcmp(keysym, "Escape") == 0)
      _mixer_popup_del(inst);