Another two mis-fixes from r9919: when we sfree(line) on exit from the

ssh.com and OpenSSH key import loops, we should also null it out so
that the cleanup path doesn't try to re-free the same pointer.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9944 cda61777-01e9-0310-a592-d414129be87e