RFR: 8194924: Checking for selection size before update

[Alexander Shaklein]

It is possible situation when `clearSelection()` is invoked during `onChange()` notify. In such case `selectedCellsSeq` is clearing and possible `IndexOutOfBoundsException` on `GenericAddRemoveChange` creation. 
So we should check it to create correct `GenericAddRemoveChange`.

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Commit messages:
 - 8194924: Checking for selection size before update

Changes: https://git.openjdk.java.net/jfx/pull/542/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=542&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8194924
  Stats: 12 lines in 2 files changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/542.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/542/head:pull/542

PR: https://git.openjdk.java.net/jfx/pull/542