RFR: 8373638: RBTree public interface does not check all input parameters for validity [v3]

[Casper Norrbin]

> Hi everyone,
> 
> The public interface for the `RBTree` assumes most input is valid, and does not check for null pointers. This could lead to potential null pointer dereferences. We should instead assert to give clearer errors in debug builds and to avoid trying to dereference these null pointers.
> 
> Testing:
> - Oracle tiers 1-3

Casper Norrbin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - move preconds to inline file
 - Merge branch 'master' into rbtree-more-asserts
 - changed asserts to precond
 - nullptr asserts

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Changes: https://git.openjdk.org/jdk/pull/28922/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28922&range=02
  Stats: 17 lines in 1 file changed: 11 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/28922.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28922/head:pull/28922

PR: https://git.openjdk.org/jdk/pull/28922