Fix assertion failure in Unix PuTTYgen exports.

The assertions I added to sshrand.c in r9930 are now justified,
because they were failing when cmdgen was used to convert a key into
either foreign private key file format - both the export functions
require random_byte() for one reason or another, and random_ref()
hadn't been called first.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10117 cda61777-01e9-0310-a592-d414129be87e