eventhandler: Fix possible crash on OBS start in Studio Mode

When OBS Studio is started in Studio Mode, if a scene collection was not
successfully found and loaded, the OBS Frontend API would, prior to
obsproject/obs-studio@577e350,
emit OBS_FRONTEND_EVENT_SCENE_CHANGED and
OBS_FRONTEND_EVENT_PREVIEW_SCENE_CHANGED anyway. This would result in
currentScene here being a nullptr, causing a crash.

For completeness, check for nullptr here so that we cannot crash if
obs_frontend_get_current_scene() returns a nullptr.

(cherry picked from commit 05e13b1)

Author: RytoEX

src/eventhandler/EventHandler_Scenes.cpp

@@ -109,6 +109,9 @@ void EventHandler::HandleCurrentProgramSceneChanged()
 {
 	OBSSourceAutoRelease currentScene = obs_frontend_get_current_scene();
 
+	if (!currentScene)
+		return;
+
 	json eventData;
 	eventData["sceneName"] = obs_source_get_name(currentScene);
 	eventData["sceneUuid"] = obs_source_get_uuid(currentScene);