image-source: Use mutex when accessing slideshow

While adding or updating files locked this mutex, the graphics thread
did not. As the update operation is not atomic, the graphics thread
might access the darray in the middle of an update, resulting in access
to freed memory (crash) if the files were updated at the same time.

plugins/image-source/obs-slideshow.c

@@ -738,8 +738,10 @@ static void ss_video_tick(void *data, float seconds)
 {
 	struct slideshow *ss = data;
 
+	pthread_mutex_lock(&ss->mutex);
+
 	if (!ss->transition || !ss->slide_time)
-		return;
+		goto finish;
 
 	if (ss->restart_on_activate && ss->use_cut) {
 		ss->elapsed = 0.0f;
@@ -748,11 +750,11 @@ static void ss_video_tick(void *data, float seconds)
 		ss->restart_on_activate = false;
 		ss->use_cut = false;
 		ss->stop = false;
-		return;
+		goto finish;
 	}
 
 	if (ss->pause_on_deactivate || ss->manual || ss->stop || ss->paused)
-		return;
+		goto finish;
 
 	/* ----------------------------------------------------- */
 	/* fade to transparency when the file list becomes empty */
@@ -779,11 +781,14 @@ static void ss_video_tick(void *data, float seconds)
 			else
 				do_transition(ss, false);
 
-			return;
+			goto finish;
 		}
 
 		obs_source_media_next(ss->source);
 	}
+
+finish:
+	pthread_mutex_unlock(&ss->mutex);
 }
 
 static inline bool ss_audio_render_(obs_source_t *transition, uint64_t *ts_out,