avformat/assenc: make sure we crawl extradata only if it's non-null

libavformat/assenc.c

@@ -53,9 +53,9 @@ static int write_header(AVFormatContext *s)
         avio_write(s->pb, avctx->extradata, avctx->extradata_size);
         if (avctx->extradata[avctx->extradata_size - 1] != '\n')
             avio_write(s->pb, "\r\n", 2);
-    }
         if (!strstr(avctx->extradata, "\n[Events]"))
             avio_printf(s->pb, "[Events]\r\nFormat: Layer, Start, End, Style, Actor, MarginL, MarginR, MarginV, Effect, Text\r\n");
+    }
     avio_flush(s->pb);
 
     return 0;