Fix two double-frees in ssh2_load_userkey().

Fix two double-frees in ssh2_load_userkey().

We should NULL out mac after freeing it, so that the cleanup code
doesn't try to free it again; also if the final key creation fails, we
should avoid freeing ret->comment when we're going to go to that same
cleanup code which will free 'comment' which contains the same pointer.

Thanks to Christopher Staite for pointing these out.