sipcall: avoid use after free on the invite session

pjsip uses a counter to delete objects when the ref counter is equals to 0.
This means that our unique_ptr on the invite will be invalid if resources are
already freed by pjproject. To avoid this, we need to increment and decrement
the counter when we respectively create and destroy our unique_ptr on the
invite session

Change-Id: Ida5c687004b91100f1c10f83e32c1a40264c775c