SSH2: fix issues with RSA key verification

Sometimes SSH servers will null pad their RSA keys. this null padding
broke Net/SSH2.php's RSA implementation (Crypt/RSA.php's implementation
works just fine). Also, the -3 was counting the initial "\0" of $h
twice so adjust it to -2.
Merge pull request #500 from terrafrost/ssh-rsa-fix

SSH2: fix issues with RSA key verification

* terrafrost/ssh-rsa-fix:
  SSH2: fix issues with RSA key verification
Merge branch 'master' of https://github.com/phpseclib/phpseclib