格納先ポインタutf16には，十分な領域をあらかじめ確保しておく必要があります。

UTF8→UTF16変換

static int utf8to16(char* utf8, int utf8_bytes, wchar_t* utf16)
{
    int i = 0, j = 0;
    while (i < utf8_bytes) {
        char ch0 = utf8[i];
        if ((ch0 & 0x80) == 0x00) {
            utf16[j++] = ch0 & 0x7F;
            i += 1;
        } else if ((ch0 & 0xE0) == 0xC0) {
            utf16[j++] = ((ch0 & 0x1F) << 6) | (utf8[i+1] & 0x3F);
            i += 2;
        } else if ((ch0 & 0xF0) == 0xE0) {
            utf16[j++] = ((ch0 & 0x0F) << 12) | ((utf8[i+1] & 0x3F) << 6) | (utf8[i+2] & 0x3F);
            i += 3;
        } else if ((ch0 & 0xF8) == 0xF0) {
            int ucs = ((ch0 & 0x07) << 18) | ((utf8[i+1] & 0x3F) << 12) |
                      ((utf8[i+2] & 0x3F) << 6) | (utf8[i+3] & 0x3F);
            utf16[j++] = 0xD800 | (ucs >> 10);
            utf16[j++] = 0xDC00 | (ucs & 0x03FF);
            i += 4;
        } else {
            break;
        }
    }
    utf16[j] = 0;
    return j;
}