drm/sched: add missing NULL check in drm_sched_get_cleanup_job v2

Otherwise we would crash if the job is not resubmitted.

v2: fix second usage of s_fence->parent as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221004132831.134986-1-christian.koenig@amd.com