fix crash in belle_sip_channel_t, when the inactive timeout timer triggers just after the listening point has cleaned out all its channels (due to linphone_core_set_network_reachable(false)).
The main loop has only one reference when this removes, so when belle_sip_channel_close() removes the parent belle_sip_source_t from the main loop, the channel is destroyed.
The call to belle_sip_source_uninit() at the end of _close() then overwrites free memory.