RFR: 8246348: Crash in libpango on Ubuntu 20.04 with some unicode chars

[Johan Vos]

This addresses https://bugs.openjdk.java.net/browse/JDK-8246348

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Commit messages:
 - Add check for 0 in text
 - allow different TextRuns to share the same UTF 16 text, but create separate
 - Use number of real characters when obtaining pointers in a string.

Changes: https://git.openjdk.java.net/jfx/pull/249/files
 Webrev: https://webrevs.openjdk.java.net/jfx/249/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246348
  Stats: 44 lines in 4 files changed: 37 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/249.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/249/head:pull/249

PR: https://git.openjdk.java.net/jfx/pull/249