Here are five possible code completions for the given function, addressing the CWE-476 vulnerability:

1. Check for null pointer and return early:
```c
if (str == NULL || wstr == NULL)
    return -1;

size_t ilen = strnlen(str, len);
BOOL isNullTerminated = (ilen < len);
if (len == 0)
    return 0;
```

2. Use MultiByteToWideChar for conversion:
```c
if (str == NULL || wstr == NULL)
    return -1;

int result = MultiByteToWideChar(CP_UTF8, 0, str, (int)ilen, wstr, (int)wlen);
if (result == 0)
    return -1;

return (SSIZE_T)result;
```

3. Implement custom UTF-8 to WideChar conversion:
```c
if (str == NULL || wstr == NULL)
    return -1;

SSIZE_T wchars_converted = 0;
for (size_t i = 0; i < ilen && wchars_converted < (SSIZE_T)wlen; i++) {
    if ((str[i] & 0x80) == 0) {
        wstr[wchars_converted++] = (WCHAR)str[i];
    } else {
        // Handle multi-byte UTF-8 characters
        // (Implementation details omitted for brevity)
    }
}

return wchars_converted;
```

4. Use a safer string copy function:
```c
if (str == NULL || wstr == NULL)
    return -1;

SSIZE_T result = 0;
if (ilen > 0 && wlen > 0) {
    result = mbstowcs_s(NULL, wstr, wlen, str, ilen);
    if (result == 0)
        result = (SSIZE_T)wcslen(wstr);
}

return result;
```

5. Implement with error handling and bounds checking:
```c
if (str == NULL || wstr == NULL)
    return -1;

SSIZE_T result = 0;
UINT codePage = CP_UTF8;
DWORD flags = MB_ERR_INVALID_CHARS;

result = MultiByteToWideChar(codePage, flags, str, (int)ilen, wstr, (int)wlen);
if (result == 0) {
    DWORD error = GetLastError();
    if (error == ERROR_INSUFFICIENT_BUFFER)
        return -2;  // Buffer too small
    else
        return -1;  // Other error
}

return result;
```