RFR: 8260046: Assert left >= right in pointer_delta() methods

[Albert Mingkun Yang]

Adding the assertion, and fixing a benign violation.

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Commit messages:
 - delta

Changes: https://git.openjdk.java.net/jdk/pull/3900/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3900&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260046
  Stats: 3 lines in 2 files changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3900.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3900/head:pull/3900

PR: https://git.openjdk.java.net/jdk/pull/3900