RFR: 8282730: LdapLoginModule throw NPE from logout method after login failure [v2]

[Weijun Wang]

> Add null-checks in all `LoginModule` implementations. It's possible that an application calls `logout` after a login failure, where most internal variables for principals and credentials are null and removing a null from the `Subject`'s principals and credentials sets will trigger a `NullPointerException`.

Weijun Wang has updated the pull request incrementally with two additional commits since the last revision:

 - null not in code
 - sean comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9348/files
  - new: https://git.openjdk.org/jdk/pull/9348/files/b3e1f541..810a0114

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9348&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9348&range=00-01

  Stats: 49 lines in 7 files changed: 8 ins; 7 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/9348.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9348/head:pull/9348

PR: https://git.openjdk.org/jdk/pull/9348