RFR: 8343507: Parallel: Fail if verify_complete finds incorrect states

[Albert Mingkun Yang]

Trivial change of replacing `log_warning` with `fatal`, because incorrect `destination_count` always indicate some problem.

Test: tier1-3

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

Commit messages:
 - pgc-fatal

Changes: https://git.openjdk.org/jdk/pull/21865/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21865&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343507
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21865.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21865/head:pull/21865

PR: https://git.openjdk.org/jdk/pull/21865