RFR: 8272579: G1: remove unnecesary null check in G1ParScanThreadStateSet::flush

[Albert Mingkun Yang]

Simple change of replacing a null-check with an assertion.

Test: hotspot_gc

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Commit messages:
 - pss-null

Changes: https://git.openjdk.java.net/jdk/pull/5145/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5145&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272579
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5145.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5145/head:pull/5145

PR: https://git.openjdk.java.net/jdk/pull/5145