[non-BE] Fix infinite CNG in ISM decoder when going to silence

Adresses part of #552 (closed) - when the input signal suddenly drops to silence, it can cause the decoder to generate comfort noise with too high level indefinitely. This minimal fix addresses the issue at the decoder by limiting the CN level to that of the dominant object for these cases (which should be per definition the object with the highest long-term energy at the encoder). This is potentially not BE to selection, but only for this special case of suddenly dropping to zero input - which is a realistic case as e.g. microphone defects or accidentally pulling the cable out or smthg could actually cause this in a real communication.