fix leak

Following code leaks memory with EV-Hiredis 0.02

``` perl
use EV::Hiredis;

my $redis = EV::Hiredis->new;
$redis->connect('127.0.0.1');

sub get_from_redis {
    $redis->get('foo', \&get_from_redis);
}

# command
$redis->set('foo' => 'bar' x 10000, sub {
    get_from_redis();
});

# start main loop
EV::run;
```

My request will fix it.