RFR: 8272579: G1: remove unnecesary null check in G1ParScanThreadStateSet::flush [v2]

[Albert Mingkun Yang]

> Simple change of replacing a null-check with an assertion.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5145/files
  - new: https://git.openjdk.java.net/jdk/pull/5145/files/301f0087..fdaf0b09

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5145&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5145&range=00-01

  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