Crash in TURN server resolution callback when ICE objects already destroyed

[nanang]

There are two synchronization issues in ICE:

1. ICE objects are destroyed while DNS resolution of TURN server is on progress, so crash will happen when the resolution callback is invoked. Stack trace:

   Exception Type:  SIGSEGV
   Exception Codes: SEGV_MAPERR at 0x321e07eb52803ca1
   Thread 17 Crashed:
   0   0x30303363 null
   1   0x10124bb40 sess_init_update + 528
   2   0x10124f9d8 turn_on_state + 1520
   3   0x101262374 turn_on_state + 112
   4   0x10125ef74 set_state + 216
   5   0x10125f74c dns_srv_resolver_cb + 64
   6   0x1011ae078 dns_callback + 3272
   7   0x1011a698c on_read_complete + 1308
   

2. Race condition between PJMEDIA transport ICE's pjmedia_transport_close() and ICE stream transport's sess_init_update():
   1. sess_init_update() thread is suspended after evaluating destroy_req that is equal to PJ_FALSE
   2. pjmedia_transport_close() is invoked (from another thread, e.g: SIP or UI thread), then transport ICE is destroyed
   3. when sess_init_update() thread is resumed and invoking callback to transport ICE, crash occurs as transport ICE is destroyed already.

Thanks Qinxi Chen and Alexei Kolomiets for the report.

[nanang]

In 5583:

Re #2009: Avoid premature destroy in TURN session by incrementing group lock ref before performing async DNS resolution.

[nanang]

In 5604:

Close #2009: Fixed issue no 2:

• Synchronizing PJMEDIA transport ICE and ICE stream transport lifetime using group lock.
• Zeroing callback pointer in ICE stream transport destructor.
• Added few pointer validity checks.