[Object] Fix a crash in Archive::child_iterator's default constructor.

To be default constructible, Archive::child_iterator needs to be able to
construct an Archive::Child with a null parent, however Archive::Child's
constructor always dereferenced its Parent argument to compute the remaining
archive size. This commit fixes Archive::Child's constructor to only do the
size calculation when the parent is non-null.

llvm-svn: 283387