Overflow memset in Dhcp.cpp

Overflow risk in `Dhcp.cpp`.
_dhcpLocalIp has only 4 byte.
Every variable should be filled separate with zeros.
```c++
void DhcpClass::reset_DHCP_lease(){
    // zero out _dhcpSubnetMask, _dhcpGatewayIp, _dhcpLocalIp, _dhcpDhcpServerIp, _dhcpDnsServerIp
    memset(_dhcpLocalIp, 0, 20);
}
```