RFR: 4200096: OffScreenImageSource.removeConsumer NullPointerException [v5]

[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.

Jeremy has updated the pull request incrementally with one additional commit since the last revision:

  4200096: updating test class header to better describe test

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13408/files
  - new: https://git.openjdk.org/jdk/pull/13408/files/1a6c1774..ff469b23

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13408&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13408&range=03-04

  Stats: 8 lines in 1 file changed: 4 ins; 1 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