term: ensure term->buf is valid

The fallowing test (reduced), submitted by @mhinz may free term->buf,
leaving the pointer dangling.

```vim
let s:buf  = -1

function! s:exit_handler()
  execute 'bdelete!' s:buf
endfunction

vnew
let s:buf = bufnr('%')
let id = termopen('sleep 1', { 'on_exit': function('s:exit_handler') })

call s:test()
```

When the buffer is known to be closing, set term->buf to NULL, and check
buf_valid() in on_refresh().

Helped-by: Marco Hinz (@mhinz)

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