lavf/apngdec: properly skip currently unsupported in-stream tags

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/apngdec.c

@@ -373,6 +373,7 @@ static int apng_read_packet(AVFormatContext *s, AVPacket *pkt)
         return 0;
     default:
         avpriv_request_sample(s, "In-stream tag=%#08X len=%"PRId64"", tag, avio_tell(pb));
+        avio_skip(pb, len + 4);
     }
 
     /* Handle the unsupported yet cases */