Verify 'called without prior match' error handling for a few methods.

.nmatches(), .matched() and .which().

src/tests/prefix.vtc

@@ -1485,3 +1485,44 @@ client c1 {
 } -run
 
 logexpect l1 -wait
+
+varnish v1 -vcl {
+	import ${vmod_selector};
+	import std;
+	backend b { .host = "${bad_ip}"; }
+
+	sub vcl_init {
+		new s = selector.set();
+		s.add("foo");
+	}
+
+	sub vcl_recv {
+		return (synth(200));
+	}
+
+	sub vcl_synth {
+		set resp.http.N = s.nmatches();
+		set resp.http.Which = s.which();
+		set resp.http.Matched = s.matched(1);
+		return (deliver);
+	}
+}
+
+logexpect l1 -v v1 -d 0 -g vxid -q "VCL_Error" {
+	expect 0 * Begin req
+	expect * = VCL_Error {^vmod selector error: s\.nmatches\(\) called without prior match$}
+	expect * = VCL_Error {^vmod selector error: s\.which\(\) called without prior match$}
+	expect * = VCL_Error {^vmod selector error: s\.matched\(\) called without prior match$}
+	expect * = End
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.http.N == "0"
+	expect resp.http.Which == "0"
+	expect resp.http.Matched == "false"
+} -run
+
+logexpect l1 -wait