id3v2: don't discard the whole tag when encountering empty frames.

While they're technically invalid, it's better to skip them and try to
read the rest of the tag.

libavformat/id3v2.c

@@ -401,13 +401,19 @@ static void ff_id3v2_parse(AVFormatContext *s, int len, uint8_t version, uint8_t
             tag[3] = 0;
             tlen = avio_rb24(s->pb);
         }
-        if (tlen <= 0 || tlen > len - taghdrlen) {
+        if (tlen < 0 || tlen > len - taghdrlen) {
             av_log(s, AV_LOG_WARNING, "Invalid size in frame %s, skipping the rest of tag.\n", tag);
             break;
         }
         len -= taghdrlen + tlen;
         next = avio_tell(s->pb) + tlen;
 
+        if (!tlen) {
+            if (tag[0])
+                av_log(s, AV_LOG_DEBUG, "Invalid empty frame %s, skipping.\n", tag);
+            continue;
+        }
+
         if (tflags & ID3v2_FLAG_DATALEN) {
             avio_rb32(s->pb);
             tlen -= 4;