cgroups: fix "uninitialized transient_len" warning

cgroups: fix "uninitialized transient_len" warning

Without this change, a build error is triggered if you compile with
-Werror=maybe-uninitialized.

 cgroups/cgfsng.c: In function 'cgfsng_monitor_enter':
 groups/cgfsng.c:1387:9: error: 'transient_len' may be used uninitialized in this function
    ret = lxc_writeat(h->cgfd_mon, "cgroup.procs", transient, transient_len);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The issue is that if handler->transient_pid is 0, then transient_len is
uninitialised but lxc_writeat(..., transient_len) still gets called.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>