lib/eudebug: Fix xe_eudebug_client_stop corner case

After `token_signal(c->p_in, CLIENT_STOP, c->pid)`, the client is
expected to end its work and exit. By the time the waitpid() is called,
the client process may be gone already, resulting in a failed assert.

Fix it by skipping the assert in case of errno being ECHILD, meaning
the process already exited.

Signed-off-by: Dominik Karol Piątkowski <dominik.karol.piatkowski@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>