Fix crash on PGP packets/armors with more than one key (RhBug:667582)

Fix crash on PGP packets/armors with more than one key (RhBug:667582)

- While OpenPGP permits arbitrary number of keys per packet/armor,
  we can't handle more than one, error out early.  The poor user
  wont get much of a clue as to what went wrong, but thats still
  better than crashing and burning.
- Return NULL from pgpPrtPubkeyParams() on errors and pass it onwards
  from pgpPrtKey() which propagates it up to callers. Besides
  the crash, this also fixes the error path from pgpNewPublicKey()
  failures.