Fix unitialized warning

This fixes two small problems with Net::EmptyPort. First, `wait_port` documentation says that `max_wait` argument is optional, but if it is omitted it gives a warning:
```
$ perl -MNet::EmptyPort -E'wait_port 1111'
Use of uninitialized value $max_wait in numeric gt (>) at /home/.../perl5/perlbrew/perls/perl-5.18.1/lib/site_perl/5.18.1/Net/EmptyPort.pm line 76.
Use of uninitialized value $max_wait in numeric gt (>) at /home/.../perl5/perlbrew/perls/perl-5.18.1/lib/site_perl/5.18.1/Net/EmptyPort.pm line 76.
```
Second, in compatibility mode it tries to compute `max_wait` as `$max_wait*$retry` which is wrong, as `$max_wait` is not defined yet, it should be `$sleep * $retry`.