Fixes a problem where a sample entry which cannot be written correctly appears to succeed, but produces an invalid file.
For example, this command:
ffmpeg -f lavfi -i sine=frequency=1000:duration=5 -codec:a ac3 -movflags +empty_moov -frag_duration 5000000 /tmp/foo.mp4
produced a file with the ac-3 sample entry, but no AC3SpecificBox (dac3) child, which is invalid according to ETSI TS 102 366.
Reviewed-by: Baptiste Coudurier <baptiste.coudurier@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>