[jdk11u-dev] RFR: 8227766: CheckUnhandledOops is broken in MemAllocator

[Aleksey Shipilev]

This fixes `-XX:+CheckUnhandledOops` verification code.

Additional testing:
 - [x] New test fails without the patch, passes with it
 - [x] Linux x86_64 fastdebug `tier1` with `-XX:+CheckUnhandledOops`
 - [ ] Linux x86_64 fastdebug `tier2` with `-XX:+CheckUnhandledOops`

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Commit messages:
 - 8227766: CheckUnhandledOops is broken in MemAllocator

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/73/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=73&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8227766
  Stats: 73 lines in 6 files changed: 54 ins; 8 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/73.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/73/head:pull/73

PR: https://git.openjdk.java.net/jdk11u-dev/pull/73