Support backend None

since varnish-cache ecef48518f3b3f4bbf28256e090bdbb5cd2b163c backends
can be NULL (as defined with backend <name> None)

src/tests/backend.vtc

@@ -4,6 +4,7 @@ varnishtest "backend() method"
 
 varnish v1 -vcl {
 	import ${vmod_selector};
+
 	backend b1 { .host = "${bad_ip}"; }
 	backend b2 { .host = "${bad_ip}"; }
 	backend b3 { .host = "${bad_ip}"; }
@@ -247,11 +248,13 @@ logexpect l1 -wait
 varnish v1 -vcl {
 	import ${vmod_selector};
 	backend b { .host = "${bad_ip}"; }
+	backend b0 None;
 
 	sub vcl_init {
 		new s = selector.set();
 		s.add("foo", backend=b);
 		s.add("bar");
+		s.add("bazz", backend=b0);
 	}
 
 	sub vcl_recv {
@@ -282,6 +285,11 @@ client c1 {
 	rxresp
 	expect resp.status == 200
 	expect resp.http.Backend == ""
+
+	txreq -hdr "Word: bazz"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.Backend == ""
 } -run
 
 logexpect l1 -wait

src/vmod_selector.c

@@ -670,7 +670,7 @@ vmod_set_backend(VRT_CTX, struct vmod_selector_set *set, VCL_INT n,
 		return (NULL);
 
 	b = set->table[idx]->backend;
-	CHECK_OBJ_NOTNULL(b, DIRECTOR_MAGIC);
+	CHECK_OBJ_ORNULL(b, DIRECTOR_MAGIC);
 	return (b);
 }