RFR: 8273969: Memory Leak on the Lambda provided to Platform.startup

[Florian Kirmaier]

Probably my most boring PR. ;)
Setting the lambda to null, after it has been used, fixes the leak.

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Commit messages:
 - JDK-8273969

Changes: https://git.openjdk.java.net/jfx/pull/626/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=626&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273969
  Stats: 49 lines in 2 files changed: 49 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/626.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/626/head:pull/626

PR: https://git.openjdk.java.net/jfx/pull/626