RFR: 8376491: ZGC: crash in __memset_evex_unaligned_erms when initializing heap using high values for -XX:ConcGCThreads

[Guanqiang Han]

Please review this change. Thanks!

**Description:**

This change fixes a crash during ZGC initialization when -XX:ConcGCThreads is set to an extremely large value.
ZThreadLocalAllocBuffer::initialize() creates a ZPerWorker<ThreadLocalAllocStats>, which uses ZValueStorage::alloc to allocate per-worker storage which size is related to ConcGCThreads
https://github.com/openjdk/jdk/blob/1161a640abe454b47de95ed73452a78535160deb/src/hotspot/share/gc/z/zThreadLocalAllocBuffer.cpp#L35-L38
https://github.com/openjdk/jdk/blob/1161a640abe454b47de95ed73452a78535160deb/src/hotspot/share/gc/z/zValue.hpp#L111
https://github.com/openjdk/jdk/blob/1161a640abe454b47de95ed73452a78535160deb/src/hotspot/share/gc/z/zValue.inline.hpp#L125-L127
https://github.com/openjdk/jdk/blob/1161a640abe454b47de95ed73452a78535160deb/src/hotspot/share/gc/z/zValue.inline.hpp#L60-L61
https://github.com/openjdk/jdk/blob/1161a640abe454b47de95ed73452a78535160deb/src/hotspot/share/gc/z/zValue.inline.hpp#L108-L110

For very large ConcGCThreads, the calculated block_size becomes huge and os::malloc() inside ZUtils::alloc_aligned_unfreeable() may fail.
https://github.com/openjdk/jdk/blob/1161a640abe454b47de95ed73452a78535160deb/src/hotspot/share/gc/z/zUtils.inline.hpp#L37-L42

The current code does not check the returned pointer and unconditionally aligns and memset()s it, which can lead to a segmentation fault.

**Fix:**

The fix adds a NULL check after os::malloc() and terminates the VM with a proper OutOfMemoryError message instead of crashing.

**Test:**

GHA

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Commit messages:
 - fix test func
 - fix bug 8376491

Changes: https://git.openjdk.org/jdk/pull/29460/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29460&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376491
  Stats: 50 lines in 2 files changed: 49 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29460.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29460/head:pull/29460

PR: https://git.openjdk.org/jdk/pull/29460