[foreign-memaccess+abi] RFR: Fix sonar issues and thread safety bug

[Per Minborg]

This PR proposes some fix of Sonar warnings and a thread safety issue.

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Commit messages:
 - Fix sonar issues and thread safety bug

Changes: https://git.openjdk.org/panama-foreign/pull/906/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=906&range=00
  Stats: 29 lines in 7 files changed: 3 ins; 8 del; 18 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/906.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/906/head:pull/906

PR: https://git.openjdk.org/panama-foreign/pull/906