vim-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.
983d83ff1c

Put the error message in window.c.
Cherry-pick a memory leak fix from Vim patch 8.2.0399.
Test still fails.

src/nvim/buffer_defs.h

@@ -532,6 +532,8 @@ 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.
   int b_ro_locked;              // Non-zero when the buffer can't be changed.
                                 // Used for FileChangedRO