fix: usage of BytesMut in extend_buf_sync

[paolobarbolini]

While auditing usage of bytes I've found this crate incorrectly uses BytesMut.

The reserve branch would never get called, because BufMut::has_remaining_mut for BytesMut only checks self.len() < usize::MAX. The reason why the code worked anyways is that chunk_mut allocates if there's no spare capacity available.

This fixes the issue and makes the code simpler to read.