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.

src/popupwin.c

@@ -1941,7 +1941,7 @@ popup_create(typval_T *argvars, typval_T *rettv, create_type_T type)
 	buf->b_p_ul = -1;	// no undo
 	buf->b_p_swf = FALSE;   // no swap file
 	buf->b_p_bl = FALSE;    // unlisted buffer
-	buf->b_locked = TRUE;
+	buf->b_locked = TRUE;	// prevent deleting the buffer
 
 	// Avoid that 'buftype' is reset when this buffer is entered.
 	buf->b_p_initialized = TRUE;