fix nil pointer dereference when taksrun is canceled

pkg/chains/formats/slsa/v1/pipelinerun/pipelinerun.go

@@ -119,7 +119,7 @@ func buildConfig(ctx context.Context, pro *objects.PipelineRunObjectV1Beta1) Bui
 
 			steps := []attest.StepAttestation{}
 			// tr.Status.TaskSpec.Steps and tr.Status.Steps should be sime size
-			if len(tr.Status.TaskSpec.Steps) != len(tr.Status.Steps) {
+			if tr.Status.TaskSpec == nil || len(tr.Status.TaskSpec.Steps) != len(tr.Status.Steps) {
 				logger.Errorf("Mismatch in number of steps for task run %s. TaskSpec steps: %d, Status steps: %d",
 					tr.Name, len(tr.Status.TaskSpec.Steps), len(tr.Status.Steps))
 				continue // Skip this task run entirely