avformat/electronicarts: If no packet has been read at the end do not treat it as if theres a packet

Fixes: Assertion failure
Fixes: 17770/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5700606668308480

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