Fix memory leak of vary string on stevedore alloc fail

If the stevedore failed the object creation, we would leak the temporary
VSB holding the computed vary string. This patch frees it.

Problem exists in 4.1 and later.

bin/varnishd/cache/cache_fetch.c

@@ -125,8 +125,12 @@ vbf_beresp2obj(struct busyobj *bo)
 	if (bo->uncacheable)
 		bo->fetch_objcore->flags |= OC_F_PASS;
 
-	if (!vbf_allocobj(bo, l))
+	if (!vbf_allocobj(bo, l)) {
+		if (vary != NULL)
+			VSB_destroy(&vary);
+		AZ(vary);
 		return (-1);
+	}
 
 	if (vary != NULL) {
 		b = ObjSetattr(bo->wrk, bo->fetch_objcore, OA_VARY, varyl,