avformat/asfdec_o: limit recursion depth in asf_read_unknown()

The threshold of 5 is arbitrary, both smaller and larger should work fine

Fixes: Stack overflow
Fixes: 50603/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-6049302564175872

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 1f1a3681)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>