Avoid using string-to-multibyte in ispell.el

Avoid using string-to-multibyte in ispell.el

* lisp/textmodes/ispell.el (ispell-get-decoded-string): Use
decode-coding-string instead. Note that decode-coding-string returns a
string that satisfies multibyte-string-p even if its input is pure
ASCII and the third argument is t, so the result of
ispell-get-decoded-string is always a multibyte string.