ppp(8): Fix various bugs in NOPAM section of auth_CheckPasswd

• pw is not initialized before use
• success is returned if crypt(3) errors

These bugs were introduced in r231994, which attempted to adopt DragonflyBSD
f4a9869feb646aafe72de6e5d61051a023a02676. The original author of the
Dragonfly change also noticed these mistakes and filed the PR.

PR: 222620
Submitted by: Lubos Boucek <bouceklubos AT gmail.com>
Obtained from: DragonflyBSD f4a9869feb646aafe72de6e5d61051a023a02676