RFR: 8373638: RBTree public interface does not check all input parameters for validity [v2]

[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 incrementally with one additional commit since the last revision:

  changed asserts to precond

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28922/files
  - new: https://git.openjdk.org/jdk/pull/28922/files/5df5c446..1c221184

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28922&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28922&range=00-01

  Stats: 14 lines in 2 files changed: 4 ins; 0 del; 10 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