[CI] CompositeBytesReferenceTests::testSlice failure

[hendrikmuhs]

seen in https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+intake/316/console

repros with:

./gradlew :server:test -Dtests.seed=24CCFF659D275216 -Dtests.class=org.elasticsearch.common.bytes.CompositeBytesReferenceTests -Dtests.method="testSlice" -Dtests.security.manager=true -Dtests.locale=es-CR -Dtests.timezone=America/Chicago -Dcompiler.java=11 -Druntime.java=8

fails with:

  1> [2018-11-27T09:08:28,448][INFO ][o.e.c.b.CompositeBytesReferenceTests] [testSlice] after test
  2> REPRODUCE WITH: ./gradlew :server:test -Dtests.seed=24CCFF659D275216 -Dtests.class=org.elasticsearch.common.bytes.CompositeBytesReferenceTests -Dtests.method="testSlice" -Dtests.security.manager=true -Dtests.locale=es-CR -Dtests.timezone=America/Chicago -Dcompiler.java=11 -Druntime.java=8
ERROR   0.35s | CompositeBytesReferenceTests.testSlice <<< FAILURES!
   > Throwable #1: java.lang.ArrayIndexOutOfBoundsException: -1
   >    at __randomizedtesting.SeedInfo.seed([24CCFF659D275216:336770BBDA019DDE]:0)
   >    at org.elasticsearch.common.bytes.CompositeBytesReference.slice(CompositeBytesReference.java:87)
   >    at org.elasticsearch.common.bytes.AbstractBytesReferenceTestCase.testSlice(AbstractBytesReferenceTestCase.java:75)
   >    at java.lang.Thread.run(Thread.java:748)

[elasticmachine]

Pinging @elastic/es-core-infra