Fix a crash when we try to validate <p></p>
Fix a crash when we try to validate <p></p>

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