Prevent crash in unpublishing presence when deleting account

[ming]

1. In pjsua_acc_del(), it calls pjsua_acc_set_registration(acc_id, PJ_FALSE), which calls pjsua_pres_unpublish(&pjsua_var.acc[acc_id], 0);
2. And then, it will call pjsua_pres_delete_acc() which also calls pjsua_pres_unpublish(acc, flags);
3. It's possible that publish_cb() in (1) is called at the same time as (2) and the callback is in the process of destroying the publish session, hence resulting in a crash.

[ming]

In 5949:

Fixed #2182: Prevent crash in unpublishing presence when deleting account

Note that this fix only covers the account deletion scenario. It doesn't prevent the race condition between deleting account in publish_cb and accessing the publish session itself, which should be solved using a locking mechanism.

Thanks to Bogdan Bogdan Chifor for the report.