Speed up g_dataset_id_dup_data
Speed up g_dataset_id_dup_data

This code is used in the property notification path, so it
better be fast. This commit removes a g_return_if_fail check and
treats the common case of just a single data element better.

Distrotech/glib,johne53/MB3Glib,mzabaluev/glib,johne53/MB3Glib,mzabaluev/glib,ieei/glib,endlessm/glib,mzabaluev/glib,ieei/glib,johne53/MB3Glib,Distrotech/glib,Distrotech/glib,johne53/MB3Glib,MathieuDuponchelle/glib,tchakabam/glib,mzabaluev/glib,ieei/glib,johne53/MB3Glib,MathieuDuponchelle/glib,tchakabam/glib,tchakabam/glib,cention-sany/glib,endlessm/glib,MathieuDuponchelle/glib,tchakabam/glib,cention-sany/glib,MathieuDuponchelle/glib,cention-sany/glib,johne53/MB3Glib,endlessm/glib,cention-sany/glib,cention-sany/glib,endlessm/glib,ieei/glib,MathieuDuponchelle/glib,tchakabam/glib,Distrotech/glib,mzabaluev/glib,Distrotech/glib,endlessm/glib,ieei/glib