imageoverlay: fix problem if overlay modifiers are 0
imageoverlay: fix problem if overlay modifiers are 0

Change-Id: I4e772657e809549473a633c822a7137a96467ea8

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