Character (multi-byte UTF-8) is destroyed when character spanning
`IO::BUF_SIZE` (4096 bytes) exist.

- Prepare file:

  ```ruby
  File.open("sample", "wb") { |f| f << "●" * 1370 }
  ```

- Before patched:

  ```ruby
  File.open("sample") { |f| a = []; while ch = f.getc; a << ch; end; p a }
  # => ["●", "●", ..., "●", "\xe2", "\x97", "\x8f", "●", "●", "●", "●"]

- After patched:

  ```ruby
  File.open("sample") { |f| a = []; while ch = f.getc; a << ch; end; p a }
  # => ["●", "●", ..., "●", "●", "●", "●", "●", "●"]