Possible crash in PUBLISH session if network connectivity is lost between two requests (thanks Nikolay Popok for the report)

[bennylp]

Crash in event publication framework (publishc.c), if network connectivity is lost between two consecutive PUBLISH requests (such as when the PUBLISH request is retried for authentication, or when second PUBLISH request was in the queue (because another one is in progress when application requests the second publication)).

Note that this does not happen when there is only one PUBLISH request to be sent.

To reproduce:

1. Put breakpoint in 401/407 handler in tsx_callback() in publishc.c
2. Send PUBLISH
3. Receive 401/407
4. Disconnect network
5. tsx_callback() will resend PUBLISH
6. pjsip_publishc_send() fails, a nested tsx_callback is called, pubc is destroyed
7. pjsip_publishc_send() returns back to tsx_callback() which will destroy the pubc once again.

[bennylp]

(In [3375]) Fixed #1164: Possible crash in PUBLISH session if network connectivity is lost between two requests (thanks Nikolay Popok for the report)