RFR: 4200096: OffScreenImageSource.removeConsumer NullPointerException

[Jeremy]

This resolves a 25 year old P4 ticket: a NullPointerException is printed to System.err needlessly.

This resolution involves confirming that an ImageConsumer is still registered before every notification.

I'll understand if this is rejected as unimportant, but I stumbled across this in the real world the other day and thought this was a simple enough bug to practice on.

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Commit messages:
 - 4200096: OffScreenImageSource.removeConsumer NullPointerException
 - 4200096: OffScreenImageSource.removeConsumer NullPointerException
 - 4200096: OffScreenImageSource.removeConsumer NullPointerException
 - Merge pull request #4 from openjdk/master
 - Merge pull request #3 from openjdk/master
 - Merge pull request #2 from openjdk/master
 - Merge pull request #1 from openjdk/master

Changes: https://git.openjdk.org/jdk/pull/13408/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13408&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4200096
  Stats: 178 lines in 2 files changed: 175 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13408.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13408/head:pull/13408

PR: https://git.openjdk.org/jdk/pull/13408