Signalled by Stefano Avallone.

The member function UdpSocketImpl::DoSendTo (Ptr<Packet> p, Address dad) is useless, as it is nothing more than a if-else switch.

Due to how and when it is called, one of the switches is not reachable - the simulation would hit an assert before.
The second one can be easily moved in the caller function (not plural).

Eliminating the function would result into a more streamlined function call.

Moreover, since the function is private, there's no harm in changing the class implementation.