Engine::flushStep: fix segmentation fault of timer

if built with `WITH_TIMER ON`, `python examples/frames.py ./resources/yaafe_check.wav` will fail with a segmentation fault, but `yaafe-engine -c ./resources/yaflow32k ./resources/yaafe_check.wav` works fine.

I wonder if `Engine::flushStep()` is called `N+1` times on each step in python module, while in pure C++ implemention(yaafe-engine), it is called only `N` times, and the last time is for writer like `CSVWriter.