RFR: 8273826: Correct Manifest file name and NPE checks [v2]

[Sean Coffey]

> Use correct manifest file name in the Manifest verifier checks. 
> Also - extra null check
> 
> The test doesn't reproduce the exact issue reported but should prevent future regressions in this area.

Sean Coffey has updated the pull request incrementally with one additional commit since the last revision:

  Make variables final

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5841/files
  - new: https://git.openjdk.java.net/jdk/pull/5841/files/a63c81ad..3d72a6d7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5841&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5841&range=00-01

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5841.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5841/head:pull/5841

PR: https://git.openjdk.java.net/jdk/pull/5841