Avoid detach from sync session when there is no sync session

src/org_pEp_jniadapter_AbstractEngine.cc

@@ -74,9 +74,11 @@ extern "C" {
 
         session = (PEP_SESSION) env->GetLongField(me, handle);
         if (session){
-            release(session);
+            if(sync_session != NULL){
                 detach_sync_session(session);
             }
+            release(session);
+        }
         else
             env->SetLongField(me, handle, jlong(0));
     }