Fix printing task names

tools/gdbinit

@@ -258,7 +258,7 @@ define starpu-all-tasks
   while $l != &all_jobs_list
     set $j = (struct _starpu_job*) (((unsigned long) $l) - ((unsigned long) &((struct _starpu_job *)0)->all_submitted))
     set $task = $j->task
-    printf "task %p %s\n", $task, $j->task->name
+    printf "task %p %s\n", $task, $task->name ? $task->name : ""
     set $l = $l->next
   end
 end