Avoid dividing 0 by 0 in StackScrollAlgorithm.

This is very likely the cause of b/233692725 (which is essentially an
error saying that translationZ is set to Float.NaN). Looked through all
the codepaths modifying translationZ, and this was the only one I could
find that could generate NaN.

Test: Not quite sure how to test it though, since I can't reproduce.
Will add some logging in a follow-up commit.
Bug: 233692725

Change-Id: Ie5db92ee714b2265ceacc366144bdadad00aaf51