synchronize pEpEngine close to avoid race condition

src/org/pEp/jniadapter/AbstractEngine.java

@@ -30,8 +30,10 @@ abstract class AbstractEngine implements AutoCloseable {
     }
 
     final public void close() {
+        synchronized (AbstractEngine.class){
             release();
         }
+    }
 
     private long keyserverThread;
     private long keyserverQueue;