Update CCEditBoxImplWp8.cpp

@wuasiam 
Here is the `cc_utf8_strlen` implementation:
```
long cc_utf8_strlen (const char * p, int max)
{
    CC_UNUSED_PARAM(max);
    if (p == nullptr)
        return -1;
    return StringUtils::getCharacterCountInUTF8String(p);
}
```