avformat/gif: use last frame duration

libavformat/gif.c

@@ -88,6 +88,8 @@ static int gif_get_delay(GIFContext *gif, AVPacket *prev, AVPacket *new)
         gif->duration = av_clip_uint16(new->pts - prev->pts);
     else if (!new && gif->last_delay >= 0)
         gif->duration = gif->last_delay;
+    else if (prev->duration)
+        gif->duration = prev->duration;
 
     return gif->duration;
 }