For -w output in -m mode, tweak the handling of bytes that do not
form valid UTF-8: treat them as "not a whitespace character" and
hence "not a word boundary" in the same way as non-printable
charact

For -w output in -m mode, tweak the handling of bytes that do not
form valid UTF-8: treat them as "not a whitespace character" and
hence "not a word boundary" in the same way as non-printable
characters and NUL bytes.

OK millert@

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fix the rest of the bug mitigated in the previous commit:
do not embark on an infinite loop
when -m is given and the file contains a NUL character;
OK millert@

minimal UTF-8 tests for wc(1)