avformat/flvdec: Check for EOF in amf_parse_object()

Fixes: Timeout (too long -> 1ms)
Fixes: 26108/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5653887668977664

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 33624f4f2e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/flvdec.c

@@ -469,6 +469,8 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream,
 
     num_val  = 0;
     ioc      = s->pb;
+    if (avio_feof(ioc))
+        return AVERROR_EOF;
     amf_type = avio_r8(ioc);
 
     switch (amf_type) {