MP4 DASH files can contain more than one copy of the "moov"
atom. Parsing it multiple times would mean that tracks, chunk offset
tables, sample to chunk tables etc. would be filled multiple times as
well.
Commit 2b5e8c86a6 made this easier to
trigger, though the problem could have been hit with earlier code,
too. Before that commit header parsing stopped as soon as the first
"moov" and "mdat" atoms were found. Multiple "moov" atoms before the
first "mdat" atom would therefore have triggered the bug, but I'm not
aware of that having ever happened.
Fixes#1877.
