RFR: JDK-8311983: ListView sometimes throws an IndexOutOfBoundsException

[Marius Hanl]

An IOOBE was thrown when scrolling up via the trough (-> `VirtualScrollBar#adjustValue`).
This happened only when it has bigger cells than the viewport. 
If the the uppermost cell with the index 0 is only visible (although not completely scrolled to the top) and then an attempt is made to scroll up again, the `VirtualFlow` will try to scroll to the next cell, subtracting index 0 by 1, resulting in -1 -> IOOBE.

The code now guards against any under or overflow.

This is technically a regression from https://bugs.openjdk.org/browse/JDK-8173321

Note: While testing with very big cells, I found out that scrolling via the trough may not work after the first time. 
This is because the `VirtualFlow` still creates 2 cells, although only one can be visible at a time (and `VirtualScrollBar` does this check, which will never be true then: `firstVisibleCell == lastVisibleCell`). This is unrelated to this fix. I can create a ticket when I have more information.

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Commit messages:
 - JDK-8311983: ListView sometimes throws an IndexOutOfBoundsException

Changes: https://git.openjdk.org/jfx/pull/1194/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1194&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311983
  Stats: 39 lines in 3 files changed: 30 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jfx/pull/1194.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1194/head:pull/1194

PR: https://git.openjdk.org/jfx/pull/1194