vrt: Perform IP to STRING conversion on the stack

Performing the conversion on the stack could lead to a buffer too small
to store the string representation of the IP address. There is no test
case because the error handling is output to stderr.

Refs #3765

Conflicts:
	bin/varnishd/cache/cache_vrt.c

bin/varnishd/cache/cache_vrt.c

@@ -597,21 +597,16 @@ VRT_r_now(VRT_CTX)
 VCL_STRING v_matchproto_()
 VRT_IP_string(VRT_CTX, VCL_IP ip)
 {
-	char *p;
-	unsigned len;
+	char buf[VTCP_ADDRBUFSIZE];
+	struct vsb vsb[1];
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	if (ip == NULL)
 		return (NULL);
-	len = WS_ReserveAll(ctx->ws);
-	if (len == 0) {
-		WS_Release(ctx->ws, 0);
-		return (NULL);
-	}
-	p = ctx->ws->f;
-	VTCP_name(ip, p, len, NULL, 0);
-	WS_Release(ctx->ws, strlen(p) + 1);
-	return (p);
+	VTCP_name(ip, buf, sizeof buf, NULL, 0);
+	WS_VSB_new(vsb, ctx->ws);
+	VSB_cat(vsb, buf);
+	return (WS_VSB_finish(vsb, ctx->ws, NULL));
 }
 
 VCL_STRING v_matchproto_()