Fix for skp chunk sizes when clips are used outside of any save

Fix for skp chunk sizes when clips are used outside of any save

[reed1, 2013-02-15 15:52:17]

1. perhaps add a brief comment inline for why we're checking for empty.
2. can we write a unit-test that triggers the bad behavior?

[robertphillips, 2013-02-15 17:26:24]

1) done
2) the Pictures test actually already does this. This was a patch to get the
bots to go green. None of the gms or skps exercise this path which is why I
missed it.

[robertphillips, 2013-03-01 14:18:23]

committed as r7917