Fix prevent issue - GetCurrent() might return null

Fix prevent issue - GetCurrent() might return null

- Internal::Stage::GetCurrent() might return null, so add null-checking
code

<Prevent message example>
[actor-impl.cpp]
2. returned_null: GetCurrent returns null. [show details]
CID 405268 (#1 of 1): Dereference null return value (NULL_RETURNS)
3. dereference: Dereferencing a pointer that might be null Dali::Internal::Stage::GetCurrent() when calling GetRenderTaskList. [show details]
1861    const RenderTaskList& taskList = Stage::GetCurrent()->GetRenderTaskList();

Change-Id: Icb306352a14c2660df89cb91261724d6f83b4983