fix(terminal): autocmds leave terminal open to wiped buffer

Problem: if buf_free_all autocommands open a terminal, it will remain open after
the buffer is freed.

Solution: close terminals again later, this time while blocking autocommands.

Did consider terminal_open checking stuff like b_locked_split instead, but
that's set during BufHidden, etc., which doesn't mean the buffer's being wiped.

src/nvim/buffer.c

@@ -902,6 +902,13 @@ void buf_freeall(buf_T *buf, int flags)
     }
   }
 
+  // Autocommands may have opened another terminal. Block them this time.
+  if (buf->terminal) {
+    block_autocmds();
+    buf_close_terminal(buf);
+    unblock_autocmds();
+  }
+
   ml_close(buf, true);              // close and delete the memline/memfile
   buf->b_ml.ml_line_count = 0;      // no lines in buffer
   if ((flags & BFA_KEEP_UNDO) == 0) {

test/functional/terminal/channel_spec.lua

@@ -34,6 +34,16 @@ describe('terminal channel is closed and later released if', function()
     feed('<Ignore>') -- add input to separate two RPC requests
     -- channel has been released after one main loop iteration
     eq(chans - 1, eval('len(nvim_list_chans())'))
+
+    command('autocmd BufWipeout * ++once let id2 = nvim_open_term(str2nr(expand("<abuf>")), {})')
+    -- channel hasn't been released yet
+    eq(
+      "Vim(call):Can't send data to closed stream",
+      pcall_err(command, [[bdelete! | call chansend(id2, 'test')]])
+    )
+    feed('<Ignore>') -- add input to separate two RPC requests
+    -- channel has been released after one main loop iteration
+    eq(chans - 1, eval('len(nvim_list_chans())'))
   end)
 
   it('opened by nvim_open_term(), closed by chanclose(), and deleted by pressing a key', function()