Small fix in low-rate-encoding-of-transient

We found an issue in transient detection when investigating the critical sample provided by FhG in their cross-check of contribution 20. We did not find any transition problem, but rather that a transition should not happen there. Now, the detection of a decay of a transient is combined with flag indicating that the previous frame had a transient. This prevents the false transient detection which caused a click in this item.