xenbus_dev: add missing error check to watch handling

So far only the watch path was checked to be zero terminated, while
the watch token was merely assumed to be.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -369,6 +369,10 @@ static int xenbus_write_watch(unsigned msg_type, struct xenbus_file_priv *u)
 		goto out;
 	}
 	token++;
+	if (memchr(token, 0, u->u.msg.len - (token - path)) == NULL) {
+		rc = -EILSEQ;
+		goto out;
+	}
 
 	if (msg_type == XS_WATCH) {
 		watch = alloc_watch_adapter(path, token);