wayland-client: handled case where pthread_setspecific failed.

wayland-client: handled case where pthread_setspecific failed.

 - The thread_data of wayland-client is managed in
  pthread specific and wl_list.
 - When pthread_setspecific fails, the returned
  error number will be printed.
 - If pthread_setspecific fails, pthread_getspecific
  in get_thread_data called next will also return null,
  but thread_data has been registered in wl_list.
 - In that case, it should be able to use thread_data in wl_list.

Change-Id: Ib4e436427715d0cc23bdcf0d8b6417212530803d
Signed-off-by: Joonbum Ko <joonbum.ko@samsung.com>