popen4 can return nil status

According to popen4 source code comments:

> On windows executing a non existent command does not raise an error
> (as in unix) so on unix we return nil instead of a status object

It means that when java is not installed on unix 84 line in compressor.rb will throw "undefined method `exitstatus' for nil:NilClass (tested on Ubuntu with no java installed). That's why I've added validation for status presence.