libobs: Handle obs_scene_add failure

Previously a NULL item would cause a crash when reading transform info.
The crash can be reproduced by e.g. editing scenes.json so that two
scenes contain each other. Example scenes.json:

{
    "current_scene": "Scene",
    "sources": [
        {
            "flags": 0,
            "id": "scene",
            "mixers": 0,
            "name": "Scene",
            "settings": {
                "items": [
                    {
                        "align": 5,
                        "bounds": {
                            "x": 1440.0,
                            "y": 900.0
                        },
                        "bounds_align": 0,
                        "bounds_type": 2,
                        "name": "Scene 2",
                        "pos": {
                            "x": 0.0,
                            "y": 0.0
                        },
                        "rot": 0.0,
                        "scale": {
                            "x": 1.0,
                            "y": 1.0
                        },
                        "visible": true
                    }
                ]
            },
            "sync": 0,
            "volume": 1.0
        },
        {
            "flags": 0,
            "id": "scene",
            "mixers": 0,
            "name": "Scene 2",
            "settings": {
                "items": [
                    {
                        "align": 5,
                        "bounds": {
                            "x": 1.0,
                            "y": 1.0
                        },
                        "bounds_align": 0,
                        "bounds_type": 0,
                        "name": "Scene",
                        "pos": {
                            "x": 854.0,
                            "y": -520.0
                        },
                        "rot": 0.0,
                        "scale": {
                            "x": 1.75,
                            "y": 1.7562724351882935
                        },
                        "visible": true
                    }
                ]
            },
            "sync": 0,
            "volume": 1.0
        }
    ]
}

libobs/obs-scene.c

@@ -344,6 +344,14 @@ static void scene_load_item(struct obs_scene *scene, obs_data_t *item_data)
 	}
 
 	item = obs_scene_add(scene, source);
+	if (!item) {
+		blog(LOG_WARNING, "[scene_load_item] Could not add source '%s' "
+		                  "to scene '%s'!",
+		                  name, obs_source_get_name(scene->source));
+		
+		obs_source_release(source);
+		return;
+	}
 
 	obs_data_set_default_int(item_data, "align",
 			OBS_ALIGN_TOP | OBS_ALIGN_LEFT);