Modify decodeUTF8Chars to gracefully treat end of input

[altavir]

To properly evaluate end of file and safely treat trailing read symbol after \r in readUTF8Line. I do not see safe way to implement it without changing how decodeUTF8Chars works. Trying to call eof() inside condition does not work because of inline nature of the method.

[fzhinkin]

We're rebooting the kotlinx-io development (see #131), all issues related to the previous versions will be closed. Consider reopening it if the issue remains (or the feature is still missing) in a new version.