MINOR: Make `tasklet_free()` safe to be called with `NULL`

Make this freeing function safe, like other freeing functions are as discussed
in GitHub issue #2126.
diff --git a/include/haproxy/task.h b/include/haproxy/task.h
index e31e9da..390db0f 100644
--- a/include/haproxy/task.h
+++ b/include/haproxy/task.h
@@ -621,6 +621,9 @@
 /* Should only be called by the thread responsible for the tasklet */
 static inline void tasklet_free(struct tasklet *tl)
 {
+	if (!tl)
+		return;
+
 	if (MT_LIST_DELETE(list_to_mt_list(&tl->list)))
 		_HA_ATOMIC_DEC(&ha_thread_ctx[tl->tid >= 0 ? tl->tid : tid].rq_total);