Problem:    "echo 'abc' > ''" returns 0 or 1, depending on 'ignorecase'.
            Checks in mb_strnicmp() for illegal and truncated bytes are
            wrong.  Should not assume that byte length is equal before case
            folding.
Solution:   Add utf_safe_read_char_adv() and utf_strnicmp(). Add a test for
            this. (Ivan Krasilnikov)