[foreign-memaccess+abi] RFR: Fix Sonar warnings in jdk.internal.foreign

[Per Minborg]

This PR proposes to do some minor fixes and cleanups for warnings that were flagged by Sonar.

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Commit messages:
 - Fix Sonar warnings in jdk.internal.foreign

Changes: https://git.openjdk.org/panama-foreign/pull/905/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=905&range=00
  Stats: 10 lines in 3 files changed: 2 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/905.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/905/head:pull/905

PR: https://git.openjdk.org/panama-foreign/pull/905