patch 8.2.2476: using freed memory when splitting window while closing buffer

Problem:    Using freed memory when using an autocommand to split a window
            while a buffer is being closed.
Solution:   Disallow splitting when the buffer has b_locked_split set.
diff --git a/src/errors.h b/src/errors.h
index e6d7ab2..ec038ec 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -353,3 +353,5 @@
 	INIT(= N_("E1157: Missing return type"));
 EXTERN char e_cannot_use_flatten_in_vim9_script[]
 	INIT(= N_("E1158: Cannot use flatten() in Vim9 script"));
+EXTERN char e_cannot_split_window_when_closing_buffer[]
+	INIT(= N_("E1159: Cannot split a window when closing the buffer"));