Bug 1057256 Part 1 - Remove NS_ENSURE_TRUE in SelectionCarets.cpp.

Remove NS_ENSURE_TRUE and NS_ENSURE_TRUE_VOID since they are deprecated,
and we do not need to print log in plain null pointer check.
Bug 1057256 Part 2 - Add null pointer check for touch and selection carets.

TouchCaret::IsDisplayable() contains more null pointer check. It covers
what original there in TouchCaret::HandleEvent().