matrix: Transfer ownership of client to pointer array
The pointer array uses a g_object_unref()
as cleanup func so we need
to add a ref'ed object to the array as client
is marked as
g_autoptr
.
This fixes a crash on shutdown in eignvalue when we unref the client we
get from the liststore returned by cm_matrix_get_clients_list()
via
g_list_model_get_item()
(which gives us a reference).