WingDings font gives "iconv(): Detected an incomplete multibyte character in input string"

Reproduction

Convert file tekst2.zip using:

        $rtf = file_get_contents('tekst2.rtf');

        $document = new Document(rtrim($rtf, "\0"));  // remove ASCII 0, the NULL-byte.
        $formatter = new HtmlFormatter('UTF-8');
        $html = $formatter->Format($document);

This will throw an exception:

iconv(): Detected an incomplete multibyte character in input string at /.../public_html/vendor/henck/rtf-to-html/src/Html/HtmlFormatter.php:375

Analysis

• On the first line "{\f1\fswiss\fcharset128 Wingdings;}" the WingDings font is defined with fcharset 128.
• fcharset 128 is the Japanese Shift-JIS multibyte font. When the first character has a code of 128 or higher, the total character is compromised of two bytes.
• HtmlFormatter.php is unware of the multibyte CP932 codepage. And calls DecodeUnicode() with only one character. iconv() then throws an exception, because only the half of the multibyte character was provided.

Start in HtmlFormatter.php around line 375 and trace DecodeUnicode() for characters from the fcharset 128 WingDings definition. Reproduce the conversion with tekst2.rtf, then verify that CP932 multibyte input no longer causes iconv() to report an incomplete character and that HTML conversion completes without the exception.