patch 8.1.1575: callbacks may be garbage collected

Problem:    Callbacks may be garbage collected.
Solution:   Set reference in callbacks. (Ozaki Kiichi, closes #4564)
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index af7aa4b..2537e8d 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -566,7 +566,7 @@
     timer_T	*timer;
     typval_T	tv;
 
-    for (timer = first_timer; timer != NULL; timer = timer->tr_next)
+    for (timer = first_timer; !abort && timer != NULL; timer = timer->tr_next)
     {
 	if (timer->tr_callback.cb_partial != NULL)
 	{