TEL-4972: create switch_core_session_get_channel_unsafe in order to avoid crash when no channel

[StefanYohansson]

the switch_core_session_get_channel has an assert that leads to a crash when the channel is not ready or already hang up which happens a lot with Push Notification calls.

see mod_telnyx_rtc PR #49

[dev-ryanc]

The reason why they added the assert is that there shouldn't be a case where a session doesn't have a channel. When you create a session, it will always create a channel. You won't be able to destroy the channel without killing the session first.

[piotrgregor]

The reason why they added the assert is that there shouldn't be a case where a session doesn't have a channel. When you create a session, it will always create a channel. You won't be able to destroy the channel without killing the session first.

@dev-ryanc assert should be disabled on production build (actually Stefan needs it to be disabled on debug build too and I can understand this, so this PR makes sense, only the content should be changed slightly)