CLEANUP: task: only perform a LIST_DEL() when the list is not empty

In tasklet_free() we unconditionally perform a LIST_DEL() even when
the list is empty, let's move the LIST_DEL() inside the matching block.
diff --git a/include/proto/task.h b/include/proto/task.h
index c876e73..1f4b52e 100644
--- a/include/proto/task.h
+++ b/include/proto/task.h
@@ -380,10 +380,10 @@
 static inline void tasklet_free(struct tasklet *tl)
 {
 	if (!LIST_ISEMPTY(&tl->list)) {
+		LIST_DEL(&tl->list);
 		task_per_thread[tid].task_list_size--;
 		_HA_ATOMIC_SUB(&tasks_run_queue, 1);
 	}
-	LIST_DEL(&tl->list);
 
 	pool_free(pool_head_tasklet, tl);
 	if (unlikely(stopping))