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/structs.h

@@ -2633,6 +2633,8 @@ struct file_buffer
     int		b_flags;	// various BF_ flags
     int		b_locked;	// Buffer is being closed or referenced, don't
 				// let autocommands wipe it out.
+    int		b_locked_split;	// Buffer is being closed, don't allow opening
+				// a new window with it.
 
     /*
      * b_ffname has the full path of the file (NULL for no name).