RFR: 8273826: Correct Manifest file name and NPE checks

[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.

-------------

Commit messages:
 - 8273826: Correct Manifest file name and NPE checks

Changes: https://git.openjdk.java.net/jdk/pull/5841/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5841&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273826
  Stats: 137 lines in 5 files changed: 127 ins; 2 del; 8 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