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

[nanang]

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

Scenario:

1. Registration refresh for account A is on progress, and somehow another registration of the same account is issued (e.g: manually by app).
2. First registration is sent successfully and account transport (pjsua_acc.auto_rereg.reg_tp) is updated properly, but the second one fails (due to another registration pending) so account transport stays NULL (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.