libluna: Fix UTF-8 encoding

libluna/src/Utf8.cpp

@@ -35,6 +35,7 @@ static Result<bool> encode_wide_char_as_utf8(wchar_t c, char* result, usize& len
     const usize utf8_len = TRY(wide_char_length_as_utf8(c));
 
     if (utf8_len > len) { return false; } // Not enough space
+    len = utf8_len;
 
     u8* buf = (u8*)result;