Join all threads before end of main

Some calls from separate threads may throw exceptions once the main()
method has returned.

server/src/main/java/com/genymobile/scrcpy/Controller.java

@@ -100,11 +100,17 @@ public class Controller {
     public void stop() {
         if (thread != null) {
             thread.interrupt();
-            thread = null;
         }
         sender.stop();
     }
 
+    public void join() throws InterruptedException {
+        if (thread != null) {
+            thread.join();
+        }
+        sender.join();
+    }
+
     public DeviceMessageSender getSender() {
         return sender;
     }

server/src/main/java/com/genymobile/scrcpy/DeviceMessageSender.java

@@ -66,7 +66,12 @@ public final class DeviceMessageSender {
     public void stop() {
         if (thread != null) {
             thread.interrupt();
-            thread = null;
+        }
+    }
+
+    public void join() throws InterruptedException {
+        if (thread != null) {
+            thread.join();
         }
     }
 }

server/src/main/java/com/genymobile/scrcpy/Server.java

@@ -117,6 +117,15 @@ public final class Server {
                 if (controller != null) {
                     controller.stop();
                 }
+
+                try {
+                    initThread.join();
+                    if (controller != null) {
+                        controller.join();
+                    }
+                } catch (InterruptedException e) {
+                    // ignore
+                }
             }
         }
     }