Ignore STUN error after pjstun_get_mapped_addr2()

[riza]

On media stream creation, the stack will call pjstun_get_mapped_addr2() and might fail if the STUN server is unreachable.

Scenario:

1. Start PJSIP with STUN server A, STUN server B (both STUN servers are reachable)
2. PJSIP will use STUN server A
3. STUN server A becomes unreachable, STUN servr B remains reachable
4. User dials a call
5. PJSIP tries to resolve call media with STUN server A, but fails. (PJLIB_UTIL_ESTUNNOTRESPOND)
6. The stack will not fallback to STUN server B nor continue without STUN.

This patch will ignore the error from pjstun_get_mapped_addr2() using pjsua_config::stun_ignore_failure.

[riza]

In 5283:

Fixed #1914: Ignore STUN error after pjstun_get_mapped_addr2().