Revert one change from r8928 and instead apply the right answer:

instead of initialising prevtf to NULL before the loop in which I
rebase it, I should take out the rebase statement completely since
it's overwritten immediately afterwards!

Spotted by clang-analyzer.

git-svn-id: svn://svn.tartarus.org/sgt/agedu@9394 cda61777-01e9-0310-a592-d414129be87e