patch 8.2.3886: can define autocmd for every event by using "au!"

Problem:    Can define autocmd for every event by using "au!".
Solution:   Check if a command is present also for "au!".
diff --git a/src/autocmd.c b/src/autocmd.c
index 789dde1..9b6a5d0 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -967,7 +967,7 @@
     last_group = AUGROUP_ERROR;		// for listing the group name
     if (*arg == '*' || *arg == NUL || *arg == '|')
     {
-	if (!forceit && *cmd != NUL)
+	if (*cmd != NUL)
 	    emsg(_(e_cannot_define_autocommands_for_all_events));
 	else
 	    for (event = (event_T)0; (int)event < NUM_EVENTS;