Don't crash when parsing a <release> with no description
Don't crash when parsing a <release> with no description

Fixes https://github.com/hughsie/appstream-glib/issues/54

ximion/appstream-glib,ikeydoherty/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib,mitya57/appstream-glib,hughsie/appstream-glib,superm1/appstream-glib,kalev/appstream-glib,kalev/appstream-glib,mitya57/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,ikeydoherty/appstream-glib,superm1/appstream-glib