Fix combo_decrypt() to throw an error for zero-length input when using a

Fix combo_decrypt() to throw an error for zero-length input when using a
padded encryption scheme.  Formerly it would try to access res[(unsigned) -1],
which resulted in core dumps on 64-bit machines, and was certainly trouble
waiting to happen on 32-bit machines (though in at least the known case
it was harmless because that byte would be overwritten after return).
Per report from Ken Colson; fix by Marko Kreen.