Continuous acquisition randomly fails to stop properly (AcqRunning) after calling stopAcq()

Following up of bliss/bliss#3477 (closed)

When calling stopAcq on a on a continuous acquisition (acq_nb_frames = 0), the DS randomly fails to get back to the AcqReady state.

Notice that the saving counter last_image_saved is less than the expected acquired frame, hence the computed status by _calcAcqStatus remains AcqRunning.