Tidy up closing logic

src/main/java/electrosphere/engine/Main.java

@@ -323,7 +323,9 @@ public class Main {
         running = false;
         if(Globals.server != null){
             Globals.server.close();
-            Globals.serverThread.interrupt();
+            if(Globals.serverThread != null){
+                Globals.serverThread.interrupt();
+            }
         }
         //shut down audio engine
         if(!Globals.HEADLESS && Globals.RUN_CLIENT){

src/main/java/electrosphere/net/server/Server.java

@@ -76,7 +76,9 @@ public class Server implements Runnable{
     
     public void close(){
         try {
-            serverSocket.close();
+            if(serverSocket != null){
+                serverSocket.close();
+            }
         } catch (IOException ex) {
             ex.printStackTrace();
         }