RFR: JDK-8263557: Possible NULL dereference in Arena::destruct_contents()

[Thomas Stuefe]

Trivial.

Sonarcloud reports a possible access to a NULL C++ object in Arena::destruct_contents():

_first->chop();

I have found no code path where this could happen but _first could conceivably be NULL after a call to Arena::reset(). Lets fix that.

GA test error on windows seems unrelated.

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Commit messages:
 - start

Changes: https://git.openjdk.java.net/jdk/pull/2994/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2994&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263557
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2994.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2994/head:pull/2994

PR: https://git.openjdk.java.net/jdk/pull/2994