Fix to previous fix, to avoid overstepping bounds of zone_depths[] array...

This is an update to the hotfix/VIC_4.1.2.h branch.  I appended a ".2" to the branch name; not sure if this is the way you want to handle this (since the original hotfix/VIC_4.1.2.h branch is gone, right?).

In unrelated testing, I found a case in which the fix to calc_root_fractions() failed.  I fixed it.  hopefully this can be released as part of 4.1.2.h.