If an error occured during freezer_wait_processes we are
freeing processes_to_wait_pids array but same time
take freezer_detach on error path to execute, which
lead in nil dereference in best case.

Simply zap dangling pointer and make sure it exist
in freezer_detach.
Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>