avformat/flvdec: Check for EOF in index reading

Fixes: Timeout
Fixes: 47992/clusterfuzz-testcase-minimized-ffmpeg_dem_LIVE_FLV_fuzzer-6020443879899136

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 ceff5d7b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>