RFR: 8339648: ZGC: Division by zero in rule_major_allocation_rate

[Matthias Baesken]

The HS jtreg test gc/stringdedup/TestStringDeduplicationAgeThreshold_ZGenerational
shows this error when running with ubsan enabled 

src/hotspot/share/gc/z/zDirector.cpp:491:74: runtime error: division by zero
    #0 0x7f09886401d4 in rule_major_allocation_rate src/hotspot/share/gc/z/zDirector.cpp:491
    #1 0x7f09886401d4 in start_gc src/hotspot/share/gc/z/zDirector.cpp:822
    #2 0x7f09886401d4 in ZDirector::run_thread() src/hotspot/share/gc/z/zDirector.cpp:912
    #3 0x7f098c1404e8 in ZThread::run_service() src/hotspot/share/gc/z/zThread.cpp:29
    #4 0x7f09897cac19 in ConcurrentGCThread::run() src/hotspot/share/gc/shared/concurrentGCThread.cpp:48
    #5 0x7f098bb46b0a in Thread::call_run() src/hotspot/share/runtime/thread.cpp:225
    #6 0x7f098b1a9881 in thread_native_entry src/hotspot/os/linux/os_linux.cpp:858

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Commit messages:
 - JDK-8339648

Changes: https://git.openjdk.org/jdk/pull/20888/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20888&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339648
  Stats: 6 lines in 1 file changed: 4 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20888.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20888/head:pull/20888

PR: https://git.openjdk.org/jdk/pull/20888