RFR: 8375227: Silent OOBE in NGGroup::renderContent

[Christopher Schnick]

This is a very basic solution to the OOBE described in the issue. While it would definitely be better to figure out the root cause, understanding the NGNode implementation is above my knowledge level atm.

The code has been like that for 10 years and no one complained, so I am not sure whether this is a deeper issue or just a small oversight when handling all possible cases.

The fix should retain the exact original workings of the method while avoiding the exception.

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Commit messages:
 - Make fix less invasive
 - Fix OOB

Changes: https://git.openjdk.org/jfx/pull/2037/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2037&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375227
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2037.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2037/head:pull/2037

PR: https://git.openjdk.org/jfx/pull/2037