The bignum code has two representations of zero, since

bn_restore_invariant (and the many loops that duplicate it) leaves a
single zero word in a bignum representing 0, whereas the constant
'Zero' does not have any data words at all. Cope with this in
bignum_cmp.

(It would be a better plan to decide on one representation and stick
with it, but this is the less disruptive fix for the moment.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@9996 cda61777-01e9-0310-a592-d414129be87e