kmsaudiomixer: Avoid invalid read in remove_adder
kmsaudiomixer: Avoid invalid read in remove_adder

Change-Id: I885ce6ffa761bca6ae3a60cd070f0f5f4afe9802

TribeMedia/kms-elements,shelsonjava/kms-elements,Kurento/kms-elements,TribeMedia/kms-elements,Kurento/kms-elements,Kurento/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements