patch 8.2.2420: too many problems with using all autocommand events

Problem:    Too many problems with using all autocommand events.
Solution:   Disallow defining an autocommand for all events.
diff --git a/src/autocmd.c b/src/autocmd.c
index f7ca41d..2f6001c 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -956,11 +956,14 @@
     last_group = AUGROUP_ERROR;		// for listing the group name
     if (*arg == '*' || *arg == NUL || *arg == '|')
     {
-	for (event = (event_T)0; (int)event < (int)NUM_EVENTS;
-					    event = (event_T)((int)event + 1))
-	    if (do_autocmd_event(event, pat,
-				 once, nested, cmd, forceit, group) == FAIL)
-		break;
+	if (!forceit && *cmd != NUL)
+	    emsg(_(e_cannot_define_autocommands_for_all_events));
+	else
+	    for (event = (event_T)0; (int)event < (int)NUM_EVENTS;
+					     event = (event_T)((int)event + 1))
+		if (do_autocmd_event(event, pat,
+				    once, nested, cmd, forceit, group) == FAIL)
+		    break;
     }
     else
     {