[foreign-memaccess+abi] RFR: Fix issue in custom arena code

[Maurizio Cimadamore]

I realized there are few issues with the custom arena code which has been added in the `Arena` javadoc as part of https://git.openjdk.org/panama-foreign/pull/821.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/822/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=822&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/822.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/822/head:pull/822

PR: https://git.openjdk.org/panama-foreign/pull/822