Add an assert that VRT_delete_backend is only called once

VRT_delete_backend() sets be->cooled to non-zero as the only place where
that is done. Assert that it is zero on entry as a check that
VRT_delete_backend isn't called multiple times.

bin/varnishd/cache/cache_backend.c

@@ -655,6 +655,7 @@ VRT_delete_backend(VRT_CTX, VCL_BACKEND *dp)
 	VRT_DisableDirector(be->director);
 	Lck_Unlock(&be->mtx);
 	Lck_Lock(&backends_mtx);
+	AZ(be->cooled);
 	be->cooled = VTIM_real() + 60.;
 	VTAILQ_REMOVE(&backends, be, list);
 	VTAILQ_INSERT_TAIL(&cool_backends, be, list);