Use correct bandwidth in ISM DTX CNG

In discrete ISM coding, objects can go into a lowrate mode which can also lower the bandwidth of the object. On SID frames, the decoder reads the SID "metadata" such as the bandwidth from the one SID into all objects. This can cause incorrect CNG if an object is coded in WB due to the lowrate mode, but the dominant object for which the SID payload has been computed has a higher bandwidth, see here:

Proposed fix is to only set the bandwidth from the SID data in the object that actually uses the SID payload to generate comfort noise. For the other objects - which run the noise estimation in the decoder - their respective bandwidth is not overwritten. Result: