image-source: Prevent random_file call when file list empty

Simple workaround to prevent integer division by zero in random_file
when add new source.

Closes jp9000/obs-studio#550

plugins/image-source/obs-slideshow.c

@@ -297,7 +297,7 @@ static void ss_update(void *data, obs_data_t *settings)
 	obs_transition_set_scale_type(ss->transition,
 			OBS_TRANSITION_SCALE_ASPECT);
 
-	if (ss->randomize)
+	if (ss->randomize && ss->files.num)
 		ss->cur_item = random_file(ss);
 	if (new_tr)
 		obs_source_add_active_child(ss->source, new_tr);