[jfx21u] RFR: 8315958: Missing range checks in GlassPasteboard

[Kevin Rushforth]

Clean backport to jfx12u.

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Commit messages:
 - Backport 7c8dd1eca1a1114acd85471cd764ebf1941dda7c

Changes: https://git.openjdk.org/jfx21u/pull/16/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=16&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315958
  Stats: 19 lines in 2 files changed: 14 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jfx21u/pull/16.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/16/head:pull/16

PR: https://git.openjdk.org/jfx21u/pull/16