Always true condition in IPAddress string-to-ipv6 conversion

Always true condition in IPAddress string-to-ipv6 conversion

https://github.com/arduino/ArduinoCore-API/blob/master/api/IPAddress.cpp#L204

`*address` is checked to be `':'` so it cannot be `'\0'`. I'm not really sure what the intended behaviour is here.

阅读 api/IPAddress.cpp 第 204 行附近的代码，并检查周围的字符串到 IPv6 转换逻辑。确定当被检查的字符不可能是 '\0' 时的预期行为，然后根据转换行为验证由此产生的修正；当条件及其预期行为一致时，该 issue 即得到解决。

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