clipboard-manager: gdk_atom_name leak in owner_change

Description

It looks like the fix for #125 (closed) introduced a leak:

153 bytes in 13 blocks are definitely lost in loss record 6,412 of 7,001
   at 0x48457A8: malloc (vg_replace_malloc.c:446)
   by 0x494ED92: g_malloc (glib/gmem.c:100)
   by 0x4969BFF: g_strdup (glib/gstrfuncs.c:323)
   by 0x54F6A56: owner_change (libxfce4ui/xfce-clipboard-manager.c:1239)
   by 0x4BC5AB4: g_closure_invoke (gobject/gclosure.c:833)
   by 0x4BE0E7D: signal_emit_unlocked_R (gobject/gsignal.c:3902)
   by 0x4BDFBCC: signal_emit_valist_unlocked (gobject/gsignal.c:3534)
   by 0x4BE0197: g_signal_emit_valist (gobject/gsignal.c:3277)
   by 0x4BE0197: g_signal_emit (???:3597)
   by 0x543DE40: _gdk_event_emit (gdk/gdkevents.c:73)
   by 0x548CEF6: gdk_event_source_dispatch (gdk/x11/gdkeventsource.c:367)
   by 0x4947590: g_main_dispatch (glib/gmain.c:3398)
   by 0x4947590: g_main_context_dispatch_unlocked (???:4249)
   by 0x49479E5: g_main_context_iterate_unlocked (glib/gmain.c:4314)
   by 0x4947D68: g_main_loop_run (glib/gmain.c:4516)
   by 0x51794C9: gtk_main (gtk/gtkmain.c:1329)
   by 0x14B15E: main (xfsettingsd/main.c:352)

Version info

4cf206b7 compiled on Arch Linux