RFR: 8288724: Prevent NullPointerException in serviceability/tmtools/jstack/DaemonThreadTest.java if jstack process fails

[Jaikiran Pai]

Can I please get a review of this test only change which updates the `DaemonThreadTest` to prevent a NPE when the `jstack` tool launched from the test fails with a non-zero exit code?

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Commit messages:
 - 8288724: Prevent NullPointerException in serviceability/tmtools/jstack/DaemonThreadTest.java if jstack process fails

Changes: https://git.openjdk.org/jdk/pull/9212/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9212&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288724
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9212.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9212/head:pull/9212

PR: https://git.openjdk.org/jdk/pull/9212