iOS application getting killed after pjsua fail to release a disconnected transport

[ming]

This is related to #1482 point 2 (spurious wakeups problem after TCP disconnection).

Scenario:

1. Registration for account A is in progress, and DNS is in the process of resolving the hostname, hence TCP transport is not created yet and account transport (pjsua_acc.auto_rereg.reg_tp) is set to NULL.
2. Transport eventually gets created, but account transport is not updated.
3. Transport disconnection occurs, normally pjsua will release the disconnected transport from the account, however as transport pointer of account A is NULL, pjsua fails to detect and release it immediately.
   • Note that the transport stays not released/destroyed until a new registration by account A (using a new transport) is issued or the account is deleted.