Crash with non-0 offset

Hi. Currently a segfault happens on cleanup time when offset != 0. Trivial example to show this:

 #!/usr/bin/perl
 use strict;
 use warnings;

 use Sys::Mmap;
 use autodie;

 system("dd=/dev/zero of=/tmp/dat bs=1k count=1");

 my $fd;
 my $data;
 open $fd, '<', "/tmp/dat";
 mmap($data, 0, PROT_READ, MAP_SHARED, $fd, 256);
