RFR: 8377316: [iOS] App crashes due to GlassThreadDataKey not initialized

[Jose Pereda]

This PR moves the pthread initialization outside the assertion, which is typically ignored as `NDEBUG` is defined in the compilation phase. Therefore a possible runtime crash is prevented if the pthread initialization is bypassed in such case.

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Commit messages:
 - Move pthread initialization outside assert

Changes: https://git.openjdk.org/jfx/pull/2065/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2065&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377316
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/2065.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2065/head:pull/2065

PR: https://git.openjdk.org/jfx/pull/2065