structure: Don't allow invalid GDates in all structures and don't allow NULL GDates in taglists
structure: Don't allow invalid GDates in all structures and don't allow NULL GDates in taglists

Some code (e.g. gstvorbistag.c) assumes non-NULL GDates in taglists and
explodes otherwise and NULL or invalid GDates don't make much sense anyway.

magcius/gstreamer,lubosz/gstreamer,cablelabs/gstreamer,Distrotech/gstreamer,magcius/gstreamer,cablelabs/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,justinjoy/gstreamer,mparis/gstreamer,ahmedammar/platform_external_gst_gstreamer,ylatuya/gstreamer,lubosz/gstreamer,surround-io/gstreamer,Lachann/gstreamer,centricular/gstreamer,lubosz/gstreamer,ensonic/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,collects/gstreamer,ahmedammar/platform_external_gst_gstreamer,centricular/gstreamer,justinjoy/gstreamer,centricular/gstreamer,magcius/gstreamer,jpxiong/gstreamer,drothlis/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,centricular/gstreamer,krichter722/gstreamer,cfoch/gstreamer,ahmedammar/platform_external_gst_gstreamer,jpxiong/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,krieger-od/gstreamer,collects/gstreamer,collects/gstreamer,cfoch/gstreamer,ahmedammar/platform_external_gst_gstreamer,Distrotech/gstreamer,StreamUtils/gstreamer,krieger-od/gstreamer,krieger-od/gstreamer,mparis/gstreamer,magcius/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,Distrotech/gstreamer,krieger-od/gstreamer,drothlis/gstreamer,Distrotech/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,krichter722/gstreamer,justinjoy/gstreamer,centricular/gstreamer,magcius/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,drothlis/gstreamer,mparis/gstreamer,lubosz/gstreamer,ensonic/gstreamer,mparis/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,justinjoy/gstreamer,collects/gstreamer,cfoch/gstreamer,lovebug356/gstreamer,mparis/gstreamer,ylatuya/gstreamer,ylatuya/gstreamer,Lachann/gstreamer,justinjoy/gstreamer,ensonic/gstreamer,Distrotech/gstreamer,krieger-od/gstreamer,cfoch/gstreamer,collects/gstreamer,lovebug356/gstreamer,ylatuya/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,cablelabs/gstreamer,drothlis/gstreamer,surround-io/gstreamer,krichter722/gstreamer,lubosz/gstreamer,lovebug356/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,ylatuya/gstreamer,ensonic/gstreamer,ahmedammar/platform_external_gst_gstreamer,surround-io/gstreamer,lovebug356/gstreamer,drothlis/gstreamer,cablelabs/gstreamer,Lachann/gstreamer