Fix memory leaks

[yak1ex]

We should do undef $call when a callback loop ends.

[yak1ex]

The memory leak is reported by https://rt.cpan.org/Ticket/Display.html?id=88628#txn-1267309 as the following code:

#!/usr/bin/perl
use strict;
use warnings;
use AnyEvent;
use AnyEvent::Digest;
use Digest::MD5;

my $file = './test.flv';

my $time = AE::timer 0, 3, sub {

my $ctx = AnyEvent::Digest->new('Digest::MD5',  unit => 655360, backend => 'aio');
    $ctx->addfile_async($file)->cb(sub {
        my $ctx = shift->recv;
        print $ctx->hexdigest ,"\n";
    });
};

AE::cv->recv;

Not only aio backend but also idle backend have a memory leak.