Keep the fetch thread busyobj pointer-ref separate from the client thread pointer-ref, and deref it on thread scheduling failure.

The code tried to deref the same pointer twice, which failed because
the VBO_DerefBusyobj() will clear the pointer when called. Separating
allows calling VBO_DerefBusyobj() for each of them.

Fixes: #1628
(cherry picked from commit 7746e30e)

Conflicts:
	bin/varnishd/cache/cache_fetch.c