fix(terminal): crash when TermClose switches back to terminal buffer

Problem:  Crash when deleting terminal buffer and TermClose switches
          back to the terminal buffer.
Solution: Set b_locked_split.

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
(cherry picked from commit ad85871ca1)

src/nvim/buffer.c

@@ -660,7 +660,10 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
   buf->b_nwindows = nwindows;
 
   if (buf->terminal) {
+    buf->b_locked_split++;
     buf_close_terminal(buf);
+    buf->b_locked_split--;
+
     // Must check this before calling buf_freeall(), otherwise is_curbuf will be true
     // in buf_freeall() but still false here, leading to a 0-line buffer.
     if (buf == curbuf && !is_curbuf) {

test/functional/autocmd/termxx_spec.lua

@@ -57,6 +57,17 @@ describe('autocmd TermClose', function()
     assert_alive()
   end)
 
+  it('TermClose switching back to terminal buffer', function()
+    local buf = api.nvim_get_current_buf()
+    api.nvim_open_term(buf, {})
+    command(('autocmd TermClose * buffer %d | new'):format(buf))
+    eq(
+      'TermClose Autocommands for "*": Vim(buffer):E1546: Cannot switch to a closing buffer',
+      pcall_err(command, 'bwipe!')
+    )
+    assert_alive()
+  end)
+
   it('triggers when fast-exiting terminal job stops', function()
     command('autocmd TermClose * let g:test_termclose = 23')
     command('terminal')