Prevent double free on Failed STUN resolution

[riza]

Failed STUN resolution when starting the library might lead to a crash caused by double free.

Scenario:

1. Set stun_ignore_failure to PJ_FALSE.
2. Turn networking OFF (Linux nework connections menu)
3. Start the application
4. pjsua_core.c !.STUN resolution failed: gethostbyname() has returned error (PJ_ERESOLVE).
5. pjsua_core.c .Error resolving STUN server: gethostbyname() has returned error (PJ_ERESOLVE) [status=70018]
6. double free, crash

Internal flow:

• pjsua_init() -> resolve_stun_server() : fail -> schedule timer to call destroy_stun_resolve_cb()
• exit pjsua_init() -> destroy_stun_resolve() -> shedule new timer to destroy STUN session. The previous timer entry hasn't been processed by worker thread
• double free and crash in busy_sleep()

Thank you to Denis Poltorak for the report.

[riza]

In r5762:

Fixed #2104: Prevent double free on Failed STUN resolution.

[riza]

In 5765:

Re #2104: Make sure that STUN session is destroyed and prevent new timer for
STUN destruction on pjsua_destroy2().