RFR: 8256397: MultipleSelectionModel throws IndexOutOfBoundException

[Florian Kirmaier]

Fixing IndexOutOfBoundsException in the MultipleSelectionModelBase and added a unit-test for it.
ticket: https://bugs.openjdk.java.net/browse/JDK-8256397
run test: `./gradlew --continue -PFULL_TEST=true controls:test --tests "*MultipleSelectionModelImplTest*"`

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Commit messages:
 - 8256397

Changes: https://git.openjdk.java.net/jfx/pull/353/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=353&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256397
  Stats: 24 lines in 2 files changed: 23 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/353.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/353/head:pull/353

PR: https://git.openjdk.java.net/jfx/pull/353