avformat/rpl: Check the number of streams

Fixes: out of memory access
Fixes: 27787/clusterfuzz-testcase-minimized-ffmpeg_dem_RPL_fuzzer-4743666463408128.fuzz

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

libavformat/rpl.c

@@ -253,6 +253,9 @@ static int rpl_read_header(AVFormatContext *s)
             error |= read_line(pb, line, sizeof(line));
     }
 
+    if (s->nb_streams == 0)
+        return AVERROR_INVALIDDATA;
+
     rpl->frames_per_chunk = read_line_and_int(pb, &error);  // video frames per chunk
     if (vst && rpl->frames_per_chunk > 1 && vst->codecpar->codec_tag != 124)
         av_log(s, AV_LOG_WARNING,