function match() fails if the regex is NULL

src/tests/match.vtc

 # looks like -*- vcl -*-
 
-varnishtest "regexp match and no-match (cf. varnish b00028.vtc)"
+varnishtest "regexp match and no-match"
 
 server s1 {
 	rxreq
 	txresp -hdr "Foo: bar" -hdr "Bar: foo" -body "1111\n"
 } -start
 
+#  cf. varnish b00028.vtc
 varnish v1 -vcl+backend {
 	import re2 from "${vmod_topbuild}/src/.libs/libvmod_re2.so";
 
@@ -68,3 +69,31 @@ client c1 {
 	expect resp.http.foo1s == "1"
 	expect resp.http.bar1s == "2"
 } -run
+
+varnish v1 -vcl {
+	import re2 from "${vmod_topbuild}/src/.libs/libvmod_re2.so";
+	backend b { .host = "${bad_ip}"; }
+
+	sub vcl_recv {
+		return(synth(200));
+	}
+
+	sub vcl_synth {
+		if (re2.match(req.http.undef, "")) {
+			set resp.http.undef = "match";
+		}
+	}
+}
+
+client c2 {
+	txreq
+	rxresp
+	expect resp.status == "200"
+	expect resp.http.undef == <undef>
+} -run
+
+logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
+	expect 0 * Begin req
+	expect * = VCL_Error "^vmod re2 error: re2.match.pattern=.<undefined>., text=...: pattern is undefined$"
+	expect * = End
+} -run

src/vmod_re2.c

@@ -596,8 +596,13 @@ vmod_match(VRT_CTX, struct vmod_priv *priv, VCL_STRING pattern,
 	int ngroups = 0;
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
-	if (pattern == NULL)
-		pattern = "";
+	if (subject == NULL)
+		subject = "";
+	if (pattern == NULL) {
+		VERR(ctx, ERR_PREFIX "pattern is undefined", "<undefined>",
+		     subject);
+		return 0;
+	}
 
 	if ((err = vre2_init(&vre2, pattern, utf8, posix_syntax, longest_match,
 			     max_mem, literal, never_nl, dot_nl, never_capture,