This function was a wrapper around RSA_check_key() in openssl, which
checks for invalid RSA private keys (like those where p or q are
composite, or where d is not the inverse of e, or where n != p*q).
We don't need a function like this in NSS, since unlike OpenSSL, NSS
won't let you import a bogus private key.

I've renamed the function and changed its return type to make it
more reasonable, and added a unit test for trying to read a key
where n != p*q.