Bugfix and verify some error cases for .match().

src/tests/match.vtc

@@ -815,3 +815,42 @@ logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
 	expect * = VCL_Error {^vmod selector failure: t\.add\(\) may only be called in vcl_init$}
 	expect * = End
 } -run
+
+varnish v1 -vcl {
+	import ${vmod_selector};
+	backend b { .host = "${bad_ip}"; }
+
+	sub vcl_init {
+		new t = selector.set();
+		t.add("foo");
+
+		new n = selector.set();
+	}
+
+	sub vcl_recv {
+		return (synth(200));
+	}
+
+	sub vcl_synth {
+		set resp.http.Undef = t.match(req.http.No-Such-Header);
+		set resp.http.Nil = n.match("foo");
+		return (deliver);
+	}
+}
+
+logexpect l1 -v v1 -d 0 -g vxid -q "VCL_Error" {
+	expect 0 * Begin req
+	expect * = VCL_Error {^vmod selector error: t\.match\(\): subject string is NULL$}
+	expect * = VCL_Error {^vmod selector error: n\.match\(\): no entries were added$}
+	expect * = End
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.http.Undef == "false"
+	expect resp.http.Nil == "false"
+} -run
+
+logexpect l1 -wait

src/vmod_selector.c

@@ -364,16 +364,17 @@ vmod_set_match(VRT_CTX, struct vmod_selector_set *set, VCL_STRING subject)
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(set, VMOD_SELECTOR_SET_MAGIC);
+
+	if (set->nmembers == 0) {
+		VERR(ctx, "%s.match(): no entries were added", set->vcl_name);
+		return (0);
+	}
 	AN(set->origo);
 
 	if (subject == NULL) {
 		VERR(ctx, "%s.match(): subject string is NULL", set->vcl_name);
 		return (0);
 	}
-	if (set->nmembers == 0) {
-		VERR(ctx, "%s.match(): no entries were added", set->vcl_name);
-		return (0);
-	}
 
 	members = set->members;
 	subj = subject;