obexd: Fix memleak when unregistering MAP event handler

obexd: Fix memleak when unregistering MAP event handler

96 bytes in 3 blocks are definitely lost in loss record 217 of 310
  at 0x4C29E84: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
  by 0x5977858: g_malloc0 (in /usr/lib/libglib-2.0.so.0.3600.3)
  by 0x433A87: map_register_event_handler (map-event.c:76)
  by 0x4324C1: set_notification_registration (map.c:1722)
  by 0x4325BB: map_probe (map.c:1801)
  by 0x42D55C: obc_session_register (session.c:862)
  by 0x42BE4B: create_callback (manager.c:100)
  by 0x42CA0D: connect_cb (session.c:281)
  by 0x4191CB: handle_response (gobex.c:949)
  by 0x4196F0: incoming_data (gobex.c:1192)
  by 0x5971DA5: g_main_context_dispatch (in /usr/lib/libglib-2.0.so.0.3600.3)
  by 0x59720F7: ??? (in /usr/lib/libglib-2.0.so.0.3600.3)