sub() function fails if the pattern is undefined

src/tests/sub.vtc

@@ -59,6 +59,10 @@ varnish v1 -vcl {
 		set resp.http.undeffallback = b.sub("b", "x", req.http.undef);
 		set resp.http.undeffallbackf
 			= re2.sub("b", "b", "x", req.http.undef);
+
+		# Undefined pattern in the function
+		set resp.http.undefpattern
+			= re2.sub(req.http.undef, "", "", "pattern undef");
 	}
 
 } -start
@@ -93,11 +97,14 @@ client c1 {
 
 	expect resp.http.undeffallback == "**SUB METHOD FAILED**"
 	expect resp.http.undeffallbackf == "**SUB FUNCTION FAILED**"
+
+	expect resp.http.undefpattern == "pattern undef"
 } -run
 
 logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
 	expect 0 * Begin req
 	expect * = VCL_Error "^vmod re2 error: b.sub..: fallback is undefined$"
 	expect * = VCL_Error "^vmod re2 error: re2.sub..: fallback is undefined$"
+	expect * = VCL_Error "^vmod re2 error: re2.sub.pattern=<undefined>, fallback=.pattern undef..: pattern is undefined$"
 	expect * = End
 } -run

src/vmod_re2.c

@@ -742,8 +742,11 @@ vmod_sub(VRT_CTX, VCL_STRING pattern, VCL_STRING text, VCL_STRING rewrite,
 		ERR(ctx, "re2.sub(): fallback is undefined");
 		return "**SUB FUNCTION FAILED**";
 	}
-	if (pattern == NULL)
-		pattern = "";
+	if (pattern == NULL) {
+		VERR(ctx, "re2.sub(pattern=<undefined>, fallback=\"%s\"): "
+		     "pattern is undefined", fallback);
+		return fallback;
+	}
 	if (text == NULL)
 		text = "";
 	if (rewrite == NULL)