Fix nil index error in makeParadoxClone

I'm not sure how/why it happened, but while testing the 1.3.2 beta cut
I got a nil index error in makeParadoxClone() while try to index on
getInven("INVEN"). My character did have an inventory, so this seems
strange. Anyway, it makes sense to check whether INVEN exists before
trying to skip it.

Now, the function checks whether INVEN exists before adding it to the
alt_nodes table.