Nullify deleted global refs to force recreation.

src/org_pEp_jniadapter_AbstractEngine.cc

@@ -218,6 +218,11 @@ extern "C" {
         env->DeleteGlobalRef(engineClass);
         env->DeleteGlobalRef(obj);
 
+        identityClass = NULL;
+        signalClass = NULL;
+        engineClass = NULL;
+        obj = NULL;
+
         session(pEp::Adapter::release);
         delete o;
     }