RFR: 8270336: [TESTBUG] Fix initialization in NonbranchyTree

[Leonid Mesnik]

The nsk.share.gc.NonbranchyTree might call initTree() twice and incorrectly reports branchiness in the case of IllegalArgumentException(...).

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Commit messages:
 - 8270336: [TESTBUG] Fix initialization in NonbranchyTree

Changes: https://git.openjdk.java.net/jdk/pull/4761/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4761&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270336
  Stats: 30 lines in 1 file changed: 4 ins; 17 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4761.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4761/head:pull/4761

PR: https://git.openjdk.java.net/jdk/pull/4761