libctf: open: fix closing of children with imported parents

libctf: open: fix closing of children with imported parents

Closing a parent dict for the last time erases all its types and strings,
which makes type and string lookups in any surviving children impossible
from then on.  Since children hold a reference to their parent, this can
only happen in ctf_dict_close of the last child, after the parent has
been closed by the caller as well.  Since DTD deletion now involves
doing type and string lookups in order to clean out the name tables,
close the parent only after the child DTDs have been deleted.