sealed-fd: Check that the supplied FD index is in the UnixFDList

The index is user provided and can be basically any number so we have to
make sure it corresponds with an FD send out of band, collected in the
UnixFDList.

src/xdp-sealed-fd.c

@@ -207,9 +207,15 @@ xdp_sealed_fd_new_from_handle (GVariant     *handle,
 
   g_return_val_if_fail (g_variant_is_of_type (handle, G_VARIANT_TYPE_HANDLE), NULL);
   g_return_val_if_fail (G_IS_UNIX_FD_LIST (fd_list), NULL);
-  g_return_val_if_fail (g_unix_fd_list_get_length (fd_list) > 0, NULL);
 
   fd_id = g_variant_get_handle (handle);
+  if (fd_id >= g_unix_fd_list_get_length (fd_list))
+    {
+      g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_INVALID_ARGUMENT,
+                           "Invalid file descriptor: index not found");
+      return NULL;
+    }
+
   fd = g_unix_fd_list_get (fd_list, fd_id, error);
   if (fd == -1)
     return NULL;