Fix a major source of ghost SIP sources

As far as I can tell, fixes #541 (closed)

With something as spooky as ghost sip sources you never know though. I have tried all the tricks I could think of to make them appear and I have not seen any with these changes.

Note that this changes a behaviour of scenic:

If you had assigned a source to a contact and that source stops writing to its shmdata for whatever reason, you'll have to reassign that source if it starts writing again.