Fix conversion of EUC-KR text (and add test suite)

- Treat truncated multi-byte characters as an error.
- Don't allow ASCII control characters to appear in the middle of a
multi-by

Fix conversion of EUC-KR text (and add test suite)

- Treat truncated multi-byte characters as an error.
- Don't allow ASCII control characters to appear in the middle of a
multi-byte character.
- There was also a bug whereby some unrecognized Unicode codepoints
would be passed through to the output unchanged when converting
Unicode to EUC-KR.

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