avformat/rpl: Check for EOF and zero framesize

Fixes: Infinite loop
Fixes: 34751/clusterfuzz-testcase-minimized-ffmpeg_dem_RPL_fuzzer-5439330800762880
Fixes: 34774/clusterfuzz-testcase-minimized-ffmpeg_dem_RPL_fuzzer-5851571660390400

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rpl.c

@@ -339,7 +339,7 @@ static int rpl_read_packet(AVFormatContext *s, AVPacket *pkt)
 
         avio_skip(pb, 4); /* flags */
         frame_size = avio_rl32(pb);
-        if (avio_seek(pb, -8, SEEK_CUR) < 0)
+        if (avio_feof(pb) || avio_seek(pb, -8, SEEK_CUR) < 0 || !frame_size)
             return AVERROR(EIO);
 
         ret = av_get_packet(pb, pkt, frame_size);