RFR: 8281553: Ensure we only require liveness from mach-nodes with barriers

[Emanuel Peter]

Adding NULL check. This also ensures we are not returning a NULL reference.

Ran tests to ensure assert is never triggered.

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Commit messages:
 - 8281553: Ensure we only require liveness from mach-nodes with barriers

Changes: https://git.openjdk.java.net/jdk/pull/7407/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7407&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281553
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7407.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7407/head:pull/7407

PR: https://git.openjdk.java.net/jdk/pull/7407