Import.pm: Don't assume {in} and {out} always exist
While working on one of the tests I did:
my $im = PublicInbox::V2Writable->new($ibx, 1);
my $im0 = $im->importer();
$im->add($mime);
Which resulted in a warning of the use of an undefined value from
atfork_child, and the test failing nastily. Inspection of the code
reveals this can happen anytime gfi_start has not been called.
So just fix atfork_child to skip closing file descriptors that have
not yet been setup.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>