job->fence is not assigned until xe_sched_job_arm(), check for
job->fence in xe_sched_job_seqno() so any usage of this function (trace
points) do not result in NULL ptr dereference. Also check job->fence
before assigning error in job trace points.

Fixes: 0ac7a2c7 ("drm/xe: Don't initialize fences at xe_sched_job_create()")
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240605055041.2082074-1-matthew.brost@intel.com