Codechange: add non-nullptr asserts in cases where it should never be nullptr

Though where similar calls are checked for nullptr as in those instances of
the use of that function it can actually return nullptr. In other words, write
down the assumption that the function never returns nullptr in an assert.

src/network/network_server.cpp

@@ -1532,6 +1532,7 @@ static void NetworkAutoCleanCompanies()
 
 	if (!_network_dedicated) {
 		const NetworkClientInfo *ci = NetworkClientInfo::GetByClientID(CLIENT_ID_SERVER);
+		assert(ci != nullptr);
 		if (Company::IsValidID(ci->client_playas)) clients_in_company[ci->client_playas] = true;
 	}
 
@@ -1918,6 +1919,7 @@ void NetworkServerDoMove(ClientID client_id, CompanyID company_id)
 	if (client_id == CLIENT_ID_SERVER && _network_dedicated) return;
 
 	NetworkClientInfo *ci = NetworkClientInfo::GetByClientID(client_id);
+	assert(ci != nullptr);
 
 	/* No need to waste network resources if the client is in the company already! */
 	if (ci->client_playas == company_id) return;