RFR: 8296854: NULL check of CTFontCopyAvailableTables return value is required

[Phil Race]

Guard against de-referencing null, although it is currently theoretical as far as I can see (more info in the bug eval)

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Commit messages:
 - 8296854

Changes: https://git.openjdk.org/jfx/pull/968/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=968&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296854
  Stats: 6 lines in 2 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/968.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/968/head:pull/968

PR: https://git.openjdk.org/jfx/pull/968