avformat/asfdec_o: Use ff_get_extradata()
Fixes: OOM
Fixes: 27240/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-5937469859823616
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>