Fix freeze when adding users on systems without access to /dev/urandom

When one attempts to create a user on a system that is not able to access /dev/urandom (such as Media Temple dv servers that run all sites in chroot jails by default) the user is created successfully but a PHP warning causes the screen to hang, making it appear as if the operation never completed.  This commit suppresses the warning and fixes the issue.  I have also tested the change on a system that *is* able to access /dev/urandom and it did not cause a problem.

It's not incredibly obvious at first glance, but if you look at the get_random_bytes() function closely you'll see that it does fully handle cases when /dev/urandom isn't accessible.  Full discussion here:  https://github.com/jenssegers/codeigniter-phpass-library/issues/1

P.S.  This is my very first pull request ever - did I do it right?  I figure it's better to start with one-character commits with big explanations, and then work my way up to big commits with one-character explanations.  Or maybe there's a happy medium somewhere in between :-)