RFR: 8332717: ZGC: Division by zero in heuristics

[Erik Österlund]

Before we have run the first major collection, the "time to perform GC" is considered not stable. We missed once such check when computing how many old GC threads we should use compared to young GC threads, resulting in a division by zero when starting the first "warmup" major collection. This division by zero is detected by UBSan. I added a check for unstable times like we have in many other places, which I have verified removes the problem.

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Commit messages:
 - 8332717: ZGC: Division by zero in heuristics

Changes: https://git.openjdk.org/jdk/pull/19768/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19768&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332717
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19768.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19768/head:pull/19768

PR: https://git.openjdk.org/jdk/pull/19768