src: store reference to call instead of call id
src: store reference to call instead of call id

Without this it's possible for the call instance to still exist, but Call::lookup(call_id) may return null, especially if the call changed to the disconnected state.

flow180/sipster,mscdex/sipster,wolf-is/sipster,wolf-is/sipster,wolf-is/sipster,mscdex/sipster,mscdex/sipster,flow180/sipster,flow180/sipster