Fix uninitialized variable peer_id

Reported by GCC

```
minetest/src/server.cpp:996:42: warning: ‘peer_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   errorstream << "ProcessData: peer=" << peer_id << e.what() << std::endl;
```

src/server.cpp

@@ -980,7 +980,7 @@ void Server::AsyncRunStep(bool initial_step)
 
 void Server::Receive()
 {
-	session_t peer_id;
+	session_t peer_id = 0;
 	try {
 		NetworkPacket pkt;
 		m_con->Receive(&pkt);