libctf: fix refcount leak in ctf_import

Calling ctf_import (fp, NULL) to cancel out a pre-existing import leaked
the refcnt increment on the parent, so it could never be freed.

New in v4.

libctf/
	* ctf-open.c (ctf_import): Do not leak a ctf_file_t ref on every
	ctf_import after the first for a given file.