Make sure that the arguments we pass regsub() came out as strings.

Fixes #921

bin/varnishtest/tests/r00921.vtc

+varnishtest "VCC type issue in regsub arg 1"
+
+server s1 {
+	rxreq 
+	expect req.http.foo == "127.0.0.1"
+	expect req.http.bar == "127.0.0.1"
+	txresp 
+} -start
+
+varnish v1 -vcl+backend { 
+	sub vcl_recv {
+		set req.http.bar = regsub(req.url, ".*", client.ip);
+		set req.http.foo = regsub(client.ip, ":.*", client.ip);
+	}
+} -start 
+
+client c1 {
+	txreq -url "/"
+	rxresp
+	expect resp.status == 200
+} -run

lib/libvcl/vcc_expr.c

@@ -454,6 +454,8 @@ vcc_Eval_Regsub(struct vcc *tl, struct expr **e, const struct symbol *sym)
 	SkipToken(tl, '(');
 
 	vcc_expr0(tl, &e2, STRING);
+	if (e2->fmt != STRING) 
+		vcc_expr_tostring(&e2, STRING);
 
 	SkipToken(tl, ',');
 	ExpectErr(tl, CSTR);
@@ -465,6 +467,8 @@ vcc_Eval_Regsub(struct vcc *tl, struct expr **e, const struct symbol *sym)
 
 	SkipToken(tl, ',');
 	vcc_expr0(tl, &e2, STRING);
+	if (e2->fmt != STRING) 
+		vcc_expr_tostring(&e2, STRING);
 	*e = vcc_expr_edit(STRING, "\v1, \v2)", *e, e2);
 	SkipToken(tl, ')');
 }