Minor bug fix introcuded by commit 69c9f3bf7dd9fe2cadd5eae0ab72ce218b78820e
Fix description:
. The case modrdn without newsuperior was not take care of.
If newparententry is NULL, set NULL to back-up original_
newparent. as well.
. Added NULL argument checking to backentry_dup and slapi_mods APIs.