avformat/flvdec: Accept last size if its off by 1

Fixes part of Ticket5648
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/flvdec.c

@@ -1140,7 +1140,7 @@ retry_duration:
 
 leave:
     last = avio_rb32(s->pb);
-    if (last != orig_size + 11 &&
+    if (last != orig_size + 11 && last != orig_size + 10 &&
         (last != orig_size || !last) && last != flv->sum_flv_tag_size &&
         !flv->broken_sizes) {
         av_log(s, AV_LOG_ERROR, "Packet mismatch %d %d\n", last, orig_size + 11);