RFR: 8375544: JfrSet::clear should not use memset

[Kim Barrett]

Please review this change to JfrSet::clear to no longer use memset to clear
the table data.  Instead use value-initialization, similarly to what is done
in the set initialization from JDK-8374445.

Testing: mach5 tier1-3

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/29327/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29327&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375544
  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29327.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29327/head:pull/29327

PR: https://git.openjdk.org/jdk/pull/29327