RFR: 8264417: ParallelCompactData::region_offset should not accept pointers outside the current region

[Albert Mingkun Yang]

Change the accepted value range for `ParallelCompactData::region_offset` to avoid the surprising result, 0.

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

Commit messages:
 - fill

Changes: https://git.openjdk.java.net/jdk/pull/3260/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3260&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264417
  Stats: 8 lines in 2 files changed: 2 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3260.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3260/head:pull/3260

PR: https://git.openjdk.java.net/jdk/pull/3260