Fix crash when deploying with implicit os name

When booted into an ostree you can deploy without passing
an --os option. That was crashing though, because
ot_admin_complete_deploy_one is called with NULL
osname but it was not handling it properly.

https://bugzilla.gnome.org/show_bug.cgi?id=710970

src/ostree/ot-admin-functions.c

@@ -77,7 +77,8 @@ ot_admin_complete_deploy_one (OstreeSysroot      *sysroot,
        * booted and merge deployments
        */
       if (opt_retain ||
-          strcmp (ostree_deployment_get_osname (deployment), osname) != 0 ||
+          (osname != NULL &&
+           strcmp (ostree_deployment_get_osname (deployment), osname) != 0) ||
           ostree_deployment_equal (deployment, booted_deployment) ||
           ostree_deployment_equal (deployment, merge_deployment))
         {