When testing whether a particular unit should be kept or discarded, it
is best to start at the very last unit of a fragment and count down,
because that way a unit that will eventually be deleted won't be
memmoved during earlier deletions; and frag/au->nb_units need only be
evaluated once in this case and the counter is automatically correct
when a unit got deleted.

It also works for double loops, i.e. when looping over all SEI messages
in all SEI units of an access unit.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>