Channel reference count correction for transfer

Crash due to a race condition between this ref correction
https://dev.iopsys.eu/voice/asterisk/-/blob/devel/channels/chan_pjsip.c#L2088-2095
and the unref under chan_voicemngr_attended_call_transfer
https://dev.iopsys.eu/voice/asterisk-chan-voicemngr/-/blob/devel/src/channels/chan_voicemngr.c?ref_type=heads#L5646-5651.
When the correction happened and finished before the unref during the transfer, which leading
the result of ref=1 after the unref, then the crash happened during hangup the channel, add an
extra correction for ref<2 could resolve this issue.