The check that an entry returned by an archiver is a direct child of
the directory being listed normalised a parent of "/" to the root, so
an entry named "/x" passed as a child of the root while "x/" and
"dir//x" were rejected.
Decide by stripping the directory prefix and checking what is left
with sanitize.Leaf, which rejects an empty name, ".", ".." and any
name containing a "/". This also covers the leading slash case.