tpl_wayland_egl_thread: Changed the result of handle_events() to assert checking.

tpl_wayland_egl_thread: Changed the result of handle_events() to assert checking.

 - If tdm_source is destroyed when the result of tdm_client_handle_events()
  is not TDM_ERROR_NONE, tdm releated functions can not be recovered.
 - Unless error is unexpected, tdm_client_handle_events() should always return ERROR_NONE,
  otherwise it should be checked with assert to make debugging easier for unknown ERROR.

Change-Id: I6681fc396772e7e4d692145cc702257b42a57fde
Signed-off-by: Joonbum Ko <joonbum.ko@samsung.com>