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

Change-Id: I4e772657e809549473a633c822a7137a96467ea8

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