arrow

GH-43949: [C++] io::BufferedInput: Fix invalid state after SetBufferSize (#44387)



### Rationale for this change

See https://github.com/apache/arrow/issues/43949

The problem is `Peek` and `Read` both calls `SetBufferSize`, however:
1. `Read` implicit says that, when `SetBufferSize` or read, the previous buffer is not being required. In this scenerio, `bytes_buffered_` is always 0, since `Read` will consume all data. And `new_buffer_size == std::min(new_buffer_size, (raw_read_bound_ - raw_read_total_))`
2. `Peek` still requires the buffer-size here. So, `bytes_buffered_` might not 0. When `Peek`, the `new_buffer_size` would less than expected, which shrinks the buffer

### What changes are included in this PR?

Update the Logic of `SetBufferSize`

1. If `bytes_buffered_ == 0`, `SetBufferSize` can discard the current buffer
2. Otherwise, `SetBufferSize` should resize minimal to `buffer_size_ + (raw_read_bound_ - raw_read_total_)`, since it should considering the current buffer

### Are these changes tested?

Yes

### Are there any user-facing changes?

Bugfix

* GitHub Issue: #43949

Lead-authored-by: mwish <maplewish117@gmail.com>
Co-authored-by: mwish <1506118561@qq.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>