basemixer: Check if the target pad is NULL
basemixer: Check if the target pad is NULL

Change-Id: Ia1edf22b8197fda6334a3dfed39a415633157d18

KurentoLegacy/gst-kurento-plugins,TribeMedia/kms-core,ESTOS/kms-core,shelsonjava/kms-core,shelsonjava/kms-core,TribeMedia/kms-core,ESTOS/kms-core,shelsonjava/kms-core,KurentoLegacy/gst-kurento-plugins,ESTOS/kms-core,shelsonjava/kms-core,ESTOS/kms-core,Kurento/kms-core,Kurento/kms-core,KurentoLegacy/gst-kurento-plugins,Kurento/kms-core,Kurento/kms-core,KurentoLegacy/gst-kurento-plugins,TribeMedia/kms-core,TribeMedia/kms-core