In openssh_read(), we shouldn't ever return SSH2_WRONG_PASSPHRASE for

In openssh_read(), we shouldn't ever return SSH2_WRONG_PASSPHRASE for
an unencrypted key. (The other import function, sshcom_read(), already
got this right.) Thanks to David Wedderwille for the report.

This is more than just an error-reporting mistake; it actually causes
Windows PuTTYgen to tight-loop on attempting to load a corrupt OpenSSH
key, because the 'wrong passphrase' return value causes the caller to
loop round and try again, but of course it knows the key is
unencrypted so it doesn't prompt for a different passphrase and just
tries again with no change...

[originally from svn r9643]