Only standalone should call carla_engine_idle

Signed-off-by: falkTX <falktx@falktx.com>

source/backend/CarlaStandalone.cpp

@@ -458,7 +458,7 @@ bool carla_engine_close(CarlaHostHandle handle)
 
 void carla_engine_idle(CarlaHostHandle handle)
 {
-    CARLA_SAFE_ASSERT_RETURN(handle->engine != nullptr,);
+    CARLA_SAFE_ASSERT_RETURN(handle->engine != nullptr && handle->isStandalone,);
 
     handle->engine->idle();