RFR: JDK-8260236: better init AnnotationCollector _contended_group

[Matthias Baesken]

The class AnnotationCollector misses to initialize _contended_group in the constructor, that should be adjusted.

See also the related sonar warning ( 1 uninitialized field at the end of the constructor call):
https://sonarcloud.io/project/issues?id=jdk&open=AXaE0c1g8L9hkQskF8qX&resolved=false&types=BUG

Thanks, Matthias

-------------

Commit messages:
 - JDK-8260236

Changes: https://git.openjdk.java.net/jdk/pull/2231/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2231&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260236
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2231.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2231/head:pull/2231

PR: https://git.openjdk.java.net/jdk/pull/2231