RFR: 8300913: ZGC: assert(to_addr != 0) failed: Should be forwarded

[Coleen Phillimore]

We thought we didn't need the keep-alive call but we do for heap walking.
Tested with failing test case locally, and tier1-4 in progress.

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

Commit messages:
 - 8300913: ZGC: assert(to_addr != 0) failed: Should be forwarded

Changes: https://git.openjdk.org/jdk/pull/12202/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12202&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300913
  Stats: 14 lines in 2 files changed: 5 ins; 8 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12202.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12202/head:pull/12202

PR: https://git.openjdk.org/jdk/pull/12202